Most people wonder at some point in their lives whether they are smarter than average. It is a completely natural question. But here is the thing — “average” is far more complicated than a single number on a piece of paper.

Intelligence is not a fixed trait that stays the same from the time you are born until the day you die. It shifts, evolves, peaks in certain areas, and deepens in others as life goes on. The average iq score by age is not one uniform number that applies to every human being at every stage of life. It is a moving picture that reflects how the human brain grows, adapts, and eventually changes with the passage of time.

In this article, we are going to break all of it down for you. We will cover what IQ actually measures, how scores typically look at each stage of life from early childhood through the senior years, what the data shows about differences between males and females, and what factors have the most influence on where a person’s score lands. By the end, you will have a full, honest picture of what intelligence looks like across a human lifetime.

Understanding IQ Before You Look at the Numbers

Before diving into age-specific data, it helps to understand what an IQ test is actually measuring and why the scoring works the way it does.

IQ stands for Intelligence Quotient. The concept originated in the early 1900s when French psychologist Alfred Binet developed a test to identify children who needed additional support in school. Over the decades, that early model evolved into the sophisticated assessments used today, including the Wechsler Intelligence Scales and the Stanford-Binet Intelligence Scales, which are the two most widely used tools in clinical and educational settings worldwide.

Modern IQ tests are not simply trivia quizzes or math tests. They measure several distinct cognitive abilities at once, including verbal comprehension (how well you understand and use language), perceptual reasoning (how well you identify patterns and relationships between objects), working memory (how effectively you hold and manipulate information in your mind), and processing speed (how quickly you can carry out mental tasks accurately).

All of these scores are combined into a single composite number. That number is always calibrated so that 100 represents the average for a given age group. This is a critical point. The average IQ score is not based on some absolute standard of intelligence — it is based on how a person performs relative to others in the same age bracket. The standard deviation is set at 15, which means roughly 68 percent of all people score between 85 and 115. Only about 2 percent of the population scores above 130, and only about 2 percent score below 70.

This also means that when you compare scores across different age groups without age adjustment, the numbers lose much of their meaning. A raw score of 120 on a test means something very different for a 12-year-old than it does for a 45-year-old. Age-adjusted scoring exists precisely to account for this.

How Does the Average IQ Score by Age Actually Change Over Time?

This is the heart of the matter. The average iq score by age follows a recognizable developmental arc, but it does not rise and fall in one clean curve. Different types of intelligence peak at different points in life, which is why the story is more nuanced than most people expect.

IQ in Early Childhood (Ages 3–7)

Young children can be assessed using tools like the Wechsler Preschool and Primary Scale of Intelligence, commonly known as the WPPSI. However, scores at this age come with a significant caveat. The developing brain is changing so rapidly during these years that IQ scores are far less stable and predictive than they become later in life.

A child who scores at the lower end of the range at age four might test considerably higher at age eight, and vice versa. Scores at this stage are treated more as developmental snapshots than as reliable indicators of future intelligence. What they can do reliably is flag potential developmental delays or early giftedness that benefits from further evaluation and support.

The average range for children at this age, when properly age-adjusted, still centers around 100. But the variability is much wider than in older age groups, reflecting the enormous individual differences in how quickly children develop language, attention, and reasoning during the preschool and early elementary years.

IQ in Middle Childhood (Ages 8–12)

By the time children reach middle childhood, their scores begin to stabilize meaningfully. The Wechsler Intelligence Scale for Children, or WISC-V, is the standard assessment tool for this age group. Scores in this range become significantly more predictive of academic trajectory and adult cognitive performance.

During these years, children begin to show measurable differences between fluid intelligence and crystallized intelligence. Fluid intelligence refers to the ability to reason through new problems without relying on prior knowledge. Crystallized intelligence refers to the store of knowledge, vocabulary, and skills a person has accumulated over time. Both are growing rapidly in middle childhood, and the brain’s architecture is being shaped in important ways by education, environment, nutrition, and stimulation.

The average iq score by age 10, when properly normed, centers at 100. Most children fall comfortably within the 90–110 band, which is considered the “average” range in virtually every classification system used by psychologists today.

IQ in Adolescence (Ages 13–17)

Adolescence is one of the most fascinating periods when it comes to cognitive development. Fluid intelligence — the raw ability to think quickly, reason abstractly, and solve novel problems — is approaching or reaching its developmental peak during these years. The teenage brain is incredibly capable in this regard, even though the prefrontal cortex, which governs impulse control and long-term decision-making, is not yet fully matured.

Research from Cambridge University’s neuroimaging studies and others has confirmed that the prefrontal cortex continues developing well into the mid-twenties. This is why teenagers can be remarkably sharp reasoners in controlled cognitive tasks while simultaneously making impulsive decisions in real-world situations — the two functions are governed by different brain systems on different developmental timelines.

The average iq score by age in this adolescent group remains centered around 100 after age adjustment, but high-performing individuals in this range often show notably strong scores in processing speed and working memory, two areas where young, plastic brains tend to excel.

IQ in Early Adulthood (Ages 18–25)

Early adulthood represents the period that cognitive scientists watch most closely. This is when fluid intelligence typically reaches its developmental ceiling for most people. Processing speed, abstract reasoning, and the ability to hold multiple pieces of information in working memory simultaneously are generally at their strongest somewhere between ages 20 and 25.

Research consistently finds the average iq score by age 25 to be at or near the highest point it will reach across a person’s lifetime — at least in terms of fluid cognitive abilities. Most large-scale normative studies place the average composite IQ score in this age range between 98 and 102, which is right where you would expect it given the 100-point baseline.

What this means practically is that young adults in their early to mid-twenties typically have the fastest learning curves, the strongest raw cognitive throughput, and the best short-term memory performance of their entire lives. It is also the age at which most cognitive assessments are standardized, making it something of a benchmark period in IQ research.

IQ in Middle Adulthood (Ages 30–55)

Here is where things get genuinely interesting — and where the popular assumption that intelligence simply declines after your twenties turns out to be an oversimplification.

Yes, fluid intelligence does begin a slow, gradual decline after the mid-twenties. Processing speed slows slightly. Working memory capacity decreases modestly. These changes are real, measurable, and well-documented in the research literature.

But here is the other side of the story: crystallized intelligence — your accumulated knowledge, vocabulary, professional expertise, and the wisdom that comes from years of lived experience — continues to grow through your thirties, forties, and fifties. In many ways, people feel subjectively smarter in midlife precisely because this accumulated intelligence compensates for the small losses in raw processing speed.

The net effect is that the average iq score by age in the middle adult years, when measured using tests that include both fluid and crystallized intelligence components, remains remarkably close to 100. The balance has simply shifted from raw speed to depth of knowledge.

IQ in Older Adults (Ages 60 and Beyond)

Cognitive aging in the senior years is a topic of tremendous research interest, and the findings are more nuanced than the stereotype of inevitable mental decline would suggest.

Fluid intelligence does decline more noticeably after 60, particularly in areas like processing speed, mental flexibility, and short-term memory. This is a well-established finding across decades of longitudinal research. However, crystallized intelligence often remains stable or even continues to improve into the early seventies for many individuals. Verbal reasoning, broad knowledge, and the ability to draw on experience to navigate complex situations do not simply fall off a cliff at retirement age.

What protects cognitive function in the senior years is also well-studied. Regular physical exercise, sustained social engagement, continued intellectual challenge, quality sleep, and management of chronic health conditions are all strongly associated with better cognitive maintenance as people age. The concept of “cognitive reserve” — essentially the resilience the brain builds through a lifetime of learning and engagement — means that education, curiosity, and mental stimulation earlier in life provide a real buffer against the effects of aging decades later.

Average IQ Score by Age — Male vs. Female: What the Research Actually Shows

One of the most frequently asked questions in this area is whether there are meaningful differences between the average iq score by age female and the average iq score by age male. The honest answer, backed by decades of research, is more nuanced than most people expect.

At the level of overall composite IQ scores, the differences between males and females across large populations are very small — so small that most researchers describe them as negligible in practical terms. The more meaningful differences emerge not in general intelligence but in specific cognitive subdomains.

What the Data Shows for Females

When looking at the average iq score by age female across the developmental lifespan, a consistent pattern emerges in verbal and language-related abilities. Girls typically develop verbal fluency, reading comprehension, and language processing skills earlier than boys do. This advantage shows up clearly in early and middle childhood assessments and is one of the most robustly replicated findings in developmental psychology.

In adolescence and young adulthood, females on average continue to show strengths in verbal reasoning, writing, and fine detail processing. Social cognition and emotional intelligence-adjacent abilities also tend to score higher on average in female populations during these years, though these traits are not always captured in standard IQ tests.

By early adulthood, the overall composite scores between males and females have converged considerably. The verbal advantage persists in many studies, but the gap in overall measured IQ is minimal.

What the Data Shows for Males

The picture for the average iq score by age male tells a somewhat different story in terms of cognitive profile rather than overall intelligence level. Males on average show slightly higher scores in spatial reasoning and mental rotation tasks — the ability to visualize objects in three dimensions and manipulate them mentally. This spatial advantage appears in childhood and persists through adulthood in most large-scale studies.

Perhaps more significant than the average difference, however, is the distribution pattern. Males tend to show greater variance in IQ scores than females do. This means that while the average scores are similar, males are overrepresented at both the very high end and the very low end of the IQ distribution. This “greater male variability” hypothesis has been examined in numerous studies and remains a topic of active scientific discussion.

By age 25, when most normative studies are centered, the cognitive profiles of males and females are broadly similar in aggregate terms. Neither group has a meaningful advantage in overall measured intelligence.

Why Differences Should Never Be Overgeneralized

The most important thing to understand about any gender-based cognitive data is that these are population-level statistical patterns, not predictions about individuals. The variation within any gender group is vastly larger than the variation between gender groups. A woman can have the strongest spatial reasoning in any room. A man can have extraordinary verbal intelligence. Individual talent, training, motivation, and opportunity matter far more than any group average.

Research by scholars like James Flynn has also shown that gender gaps in specific cognitive areas have narrowed significantly over the past several decades as educational opportunities have equalized. This tells us that environment, socialization, and cultural expectations play a significant role in shaping these patterns — they are not simply hardwired outcomes.

What Actually Influences Where IQ Scores Land

Understanding the average iq score by age also means understanding what pushes individual scores higher or lower. The answer involves a combination of factors that are both fixed and modifiable.

Genetics and Heredity play a substantial role. Twin and adoption studies have consistently estimated the heritability of IQ at somewhere between 50 and 80 percent in adults. But heritability is not destiny. Genes set a range of potential, not a fixed ceiling. The environment determines how much of that potential is realized.

Education and Stimulating Environments are among the strongest environmental predictors of IQ. Access to quality early education, exposure to books and complex ideas, and opportunities for intellectual challenge all show well-documented links to cognitive development. Children raised in enriched, stimulating environments consistently outperform those from impoverished ones — regardless of genetic potential.

Nutrition and Physical Health matter more than most people realize. Iodine deficiency alone remains one of the world’s leading preventable causes of reduced cognitive development. Omega-3 fatty acids, adequate protein, and a broad range of micronutrients all play supporting roles in brain function. Physical exercise, particularly aerobic activity, has been shown to support hippocampal growth and memory retention at every age.

Mental Health and Chronic Stress also leave measurable marks on IQ scores. Prolonged exposure to cortisol — the stress hormone — negatively affects prefrontal cortex function and working memory. Untreated anxiety, depression, or trauma can temporarily suppress measured cognitive performance. Sleep quality is another major factor: even a week of consistently poor sleep measurably impairs processing speed and reasoning in otherwise healthy adults.

The Limits of IQ: What a Number Doesn’t Capture

It would be a disservice to discuss the average iq score by age without being honest about what IQ tests do not measure. IQ captures a meaningful and useful slice of cognitive ability. It does not capture the whole picture.

Howard Gardner’s Theory of Multiple Intelligences argued that human intelligence encompasses far more than what standardized tests measure — including musical ability, bodily-kinesthetic intelligence, interpersonal skill, and naturalist intelligence, among others. Robert Sternberg’s Triarchic Theory similarly argued for analytical, creative, and practical intelligences as three distinct dimensions that conventional IQ tests largely ignore.

Real-world success — in careers, relationships, leadership, and life satisfaction — correlates imperfectly with measured IQ. Research consistently shows that traits like grit, emotional regulation, social intelligence, and perseverance are powerful independent predictors of life outcomes that IQ scores simply do not account for. A person with an IQ of 105 who is deeply motivated, emotionally resilient, and socially skilled will often outperform a person with an IQ of 130 who lacks those qualities.

IQ tests also carry well-documented cultural limitations. Standardized assessments can inadvertently reflect the cultural experiences and linguistic norms of the populations on which they were developed. People from non-Western backgrounds, lower-income communities, or non-English-speaking homes may be disadvantaged by aspects of test design that have nothing to do with their actual cognitive ability.

Practical Ways to Nurture Intelligence Across the Lifespan

One of the most reassuring findings in cognitive science is that human intelligence is far more malleable than previously believed. Here is what the research supports at each life stage.

For parents of young children, the most effective investments are simple and low-cost: reading aloud daily, encouraging imaginative play, ensuring adequate nutrition, and providing a warm and stimulating home environment. These factors show some of the strongest links to healthy cognitive development in the early years.

For teenagers, the priorities shift slightly. Encouraging academic challenge, physical activity, adequate sleep, and diverse learning experiences all support the brain during its final major growth period. Reducing chronic stress and ensuring strong social connections also matter significantly during adolescence.

For adults in their twenties through forties, continuing to learn new skills is one of the most powerful things a person can do for long-term cognitive health. Learning a new language, taking up a musical instrument, pursuing a demanding hobby, or engaging in complex problem-solving at work all stimulate the kind of neuroplasticity that keeps the brain sharp.

For those in their fifties and beyond, the research points to social engagement, physical exercise, cognitive stimulation, and management of cardiovascular health as the key protective factors. Loneliness and physical inactivity are two of the strongest risk factors for cognitive decline in older adults — and both are modifiable with intentional effort.

Final Thoughts on Average IQ Score by Age

The story of the average iq score by age is not the flat, simple narrative that many people assume. Intelligence is not fixed, and it is not one-dimensional. Different cognitive abilities rise and fall at different points across the human lifespan. Fluid intelligence peaks in early adulthood and gradually declines. Crystallized intelligence grows through the middle decades and holds its ground well into later life. The balance between the two changes constantly, but the net result is that most healthy adults maintain meaningful and effective cognitive functioning across their entire lives.

Differences between males and females in IQ are largely a matter of cognitive profile rather than overall level. Environmental factors — education, nutrition, health, stimulation, and stress — shape where individual scores land within the range that genetics makes possible. And no single IQ score captures the full scope of what it means to be intelligent, capable, curious, or successful as a human being.

FAQ 1 What is the average IQ score by age for children and adults?

The average IQ score by age is anchored at 100 for every age group without exception. This is because IQ tests use age-based norming, meaning your raw score is compared only to others in your exact age bracket. Whether you are 8 or 80, scoring 100 means you performed at the median level for your peers. Roughly 68 percent of all people fall between 85 and 115, which is considered the normal range.

FAQ 2 At what age does IQ peak?

Fluid intelligence — the ability to reason quickly, solve new problems, and process information at speed — typically peaks between the ages of 18 and 25. However, crystallized intelligence, which reflects accumulated knowledge, vocabulary, and experience, continues to grow through the thirties, forties, and even into the sixties for many people. So depending on which type of intelligence you measure, “peak” can mean very different things across the lifespan.

FAQ 3 What is the average IQ score by age 25?

Research and large-scale normative studies consistently show the average IQ score by age 25 sits between 98 and 102, which is right at the established baseline of 100. This age range is particularly significant because the prefrontal cortex — the brain region responsible for reasoning and decision-making — finishes its development around the mid-twenties. Data from the International IQ Test (2026), based on over 500,000 participants globally, confirms that average scores peak between ages 18 and 29 before gradually declining.

FAQ 4 Does IQ decrease as you get older?

It depends on which type of intelligence you are measuring. Fluid intelligence — covering processing speed, working memory, and abstract reasoning — does begin a gradual decline starting in the late twenties. However, crystallized intelligence, which includes language ability, general knowledge, and expertise built over years, typically grows well into middle age. Because IQ tests are age-normed, this natural decline does not necessarily lower your IQ score, since you are always compared to others your own age experiencing the same changes.

FAQ 5 What is the average IQ score by age for a 7-year-old?

For a 7-year-old, the average IQ score by age is 100 when measured by a properly age-normed test such as the Wechsler Intelligence Scale for Children (WISC-V). This does not mean a 7-year-old is as cognitively capable as a 30-year-old with a score of 100 — it simply means the child performed at the median level for their specific age group. Scores in the 90–110 range are considered normal for children aged 7 to 13.

FAQ 6 What is the average IQ score by age for a 10-year-old?

Like all age groups, a 10-year-old has an age-adjusted average IQ of 100 by test design. By middle childhood, IQ scores begin to stabilize and become more predictive of future cognitive performance than scores taken in the preschool years. Most 10-year-olds fall within the 90–110 average range on assessments like the WISC-V. Scores at this age are more reliable indicators of academic trajectory than those taken before age 6 or 7.

FAQ 7 What is the average IQ for a 16-year-old?

Some studies report the average IQ score for 16-year-olds as approximately 108 on non-age-adjusted measures. On properly age-normed tests, however, the average remains 100, since scores are always calibrated relative to same-age peers. The slightly elevated figure of 108 seen in some data reflects that teenagers at age 16 are near the top of their fluid intelligence development curve, with processing speed and working memory approaching their lifetime peak.

FAQ 8 What is considered a high IQ score by age?

At any age, a score above 115 is considered above average, placing someone in approximately the top 16 percent of their age group. A score of 130 or higher is classified as gifted or very superior intelligence, placing someone in the top 2 percent. Scores above 140 are sometimes described as genius-level, though they are extremely rare. These classifications apply consistently regardless of the person’s age, since all scores are benchmarked against age peers.

FAQ 9 Is there a difference in average IQ score by age between males and females?

At the level of overall composite IQ scores, the difference between males and females is very small and considered statistically negligible by most researchers. The more notable differences appear in cognitive sub-profiles: females tend to score higher on average in verbal fluency and language-related tasks, while males tend to score slightly higher in spatial reasoning. Both genders converge closely on overall average IQ by early adulthood, and individual variation within each group far exceeds any group-level difference.

FAQ 10 What is the average IQ score by age for females?

For females, average IQ scores across all age groups remain centered at 100 in the same way they do for males, since IQ is always a relative measure. Where females show a consistent advantage is in verbal reasoning, reading comprehension, and language processing — especially noticeable in childhood and adolescence. Some studies suggest the average composite IQ score in adult females is approximately 98, which is statistically very close to the global male average and well within the margin of normal variation.

FAQ 11 What is the average IQ score by age for males?

Males also average 100 across all age groups when IQ is properly age-normed. Some studies place the average composite score for adult males at approximately 102, slightly above the female average of 98 — a difference so small it has little practical meaning. What is more notable is that males show greater variance in IQ scores overall, meaning males are more heavily represented at both the high-gifted end (130+) and the significantly below-average end (below 70) than females are.

FAQ 12 How reliable are IQ scores in young children under age 6?

IQ scores measured before age 6 are significantly less stable and less predictive than those taken later in childhood. The brain is developing so rapidly in the preschool years that a score taken at age 4 can shift by 10 or more points by age 8 — and that is entirely normal. Scores in early childhood are most useful as developmental screening tools rather than as fixed indicators of future intelligence. Formal, reliable IQ testing generally begins at age 2 years and 6 months using tools like the WPPSI-IV.

FAQ 13 What is the Flynn Effect and how does it relate to average IQ scores by age?

The Flynn Effect, named after political scientist James R. Flynn, describes the documented rise in average IQ scores across populations over the 20th century — approximately 3 points per decade in many countries. Researchers attribute this to improvements in education quality, better nutrition, reduced exposure to environmental toxins, and greater familiarity with abstract thinking. However, more recent studies suggest this trend has slowed or even reversed in some developed countries since the 1990s, indicating that the gains were largely environmental rather than genetic.

FAQ 14 Can IQ change significantly during adolescence?

Yes. Research published in the journal Nature found that teenagers who took IQ tests in early adolescence improved their scores by an average of 20 points four years later — a substantial shift. This demonstrates that IQ is not fully fixed during the teenage years. The adolescent brain remains highly plastic and responsive to educational stimulation, learning experiences, and environmental enrichment. Scores tend to stabilize significantly by late adolescence, and by early adulthood, IQ becomes one of the most stable psychological traits a person has.

FAQ 15 What is the average IQ score for adults over 60?

Adults over 60 continue to average 100 on age-normed IQ tests, since the norming process always compares them to other people in their age group. What changes in the senior years is the underlying cognitive profile: fluid intelligence and processing speed decline more noticeably, while vocabulary and crystallized knowledge often hold steady or even improve into the early seventies. Factors like regular physical exercise, social engagement, cognitive stimulation, and good cardiovascular health are consistently associated with better-maintained IQ scores in older adults.

FAQ 16 What factors have the biggest influence on average IQ score by age?

The factors with the most documented influence on average IQ score by age include genetics (which accounts for an estimated 50–80 percent of IQ variation in adults), quality of education, nutritional status (particularly during early brain development), chronic stress levels, sleep quality, and access to cognitively stimulating environments. Iodine deficiency alone remains one of the world’s leading preventable causes of lower cognitive development. Physical activity and mental health management also play measurable roles across all age groups.

FAQ 17 What IQ score is considered average for an adult?

For adults of any age, an IQ score between 90 and 109 is classified as average or normal intelligence by most clinical and psychometric standards. A score of exactly 100 means the person performed at the exact median for their age group — better than 50 percent of peers. Scores from 110 to 119 are considered above average, and scores from 80 to 89 are classified as low average. These ranges apply consistently across all adult age groups because IQ scoring is always relative to same-age peers.

FAQ 18 Does education affect average IQ scores over time?

Yes, education has a well-documented and meaningful effect on measured IQ. Access to quality schooling strengthens both fluid intelligence (through problem-solving practice and abstract reasoning tasks) and crystallized intelligence (through accumulation of knowledge and vocabulary). Longitudinal studies show that additional years of schooling are associated with higher IQ scores, and early childhood enrichment programs have produced documented cognitive gains in low-income populations. Education also plays a strong protective role against age-related cognitive decline in older adults.

FAQ 19 Is an IQ of 100 actually smart?

A score of 100 means you performed at the exact median for your age group — which, by definition, makes it perfectly average rather than especially smart or below expectations. Approximately half of all people score above 100 and half score below. Whether 100 is “smart enough” depends entirely on context. For most everyday tasks, professional roles, and academic work, an IQ of 100 is entirely sufficient. Research also consistently shows that traits like motivation, emotional regulation, and perseverance matter as much as raw IQ for real-world outcomes.

FAQ 20 How is the average IQ score by age calculated and what test is used?

IQ scores are calculated by administering a standardized test to a large, representative sample of people across different ages. The average raw score for each age group is then mathematically adjusted to equal 100, with a standard deviation of 15. This process is called norming. The most widely used tests are the Wechsler Adult Intelligence Scale (WAIS) for adults, the Wechsler Intelligence Scale for Children (WISC-V) for ages 6 to 16, and the Wechsler Preschool and Primary Scale of Intelligence (WPPSI) for children aged 2 to 7.

FAQ 21 Can adults improve their IQ score after age 25?

While raw fluid intelligence is difficult to increase significantly in adulthood, overall cognitive performance can be meaningfully maintained and supported through targeted habits. Learning new skills (especially languages or musical instruments), regular aerobic exercise, quality sleep, stress management, and sustained social engagement all help preserve the cognitive abilities that IQ tests measure. Some studies show improvements in specific cognitive sub-scores through practice, though these gains do not always transfer broadly to overall IQ.

FAQ 22 What is the average IQ score by age in the United States?

The overall average IQ in the United States is approximately 97 to 100, depending on the source and testing method used — placing it among the top 30 countries globally. Like all countries, U.S. IQ averages are structured so that 100 represents the age-adjusted median. Recent data suggests that American IQ scores have declined slightly since 1995, particularly among younger age groups, a trend researchers attribute to environmental and educational factors rather than genetic changes.

FAQ 23 What does a high IQ score in childhood predict about adult intelligence?

IQ scores taken in middle childhood (ages 8 to 12) show moderate to strong correlations with adult IQ scores, with correlations typically ranging from 0.60 to 0.80. One landmark study found a significant correlation between IQ measured at age 11 and IQ measured at age 90 — suggesting that relative cognitive standing established in childhood tends to persist across a lifetime. However, individual trajectories can shift meaningfully based on education, environment, health, and life experiences, so childhood scores are not destiny.

FAQ 24 What is the difference between IQ and cognitive ability — and why does it matter for age comparisons?

IQ is a standardized score that reflects how a person’s cognitive performance compares to their age peers. Cognitive ability refers to the actual underlying mental capabilities — things like processing speed, memory, reasoning, and knowledge — that IQ tests attempt to measure. As people age, their cognitive abilities change substantially, but their IQ scores may remain stable because they are always compared to others of the same age going through the same changes. This distinction is crucial: a stable IQ score does not mean a stable brain.