First: IQ is always 100 at every age — by design
Before looking at any age-based data, one critical point needs to be clear: a person's IQ score is always relative to people of their own age. IQ tests are normed by age group — 100 always means "exactly average for people this age." A 10-year-old and a 60-year-old can both score 100, but they are being measured against completely different reference groups.
So strictly speaking, "average IQ by age" is 100 at every age — because the tests are designed that way. What actually changes is the underlying cognitive performance that earns a score of 100: the tasks a typical 10-year-old can complete at average level are different from what a typical 30-year-old does at average level. The scale stays fixed; what counts as average shifts as the population changes.
What this article is really about is something more interesting: how the cognitive abilities that IQ tests measure change across the lifespan — which ones grow, which ones decline, when each peaks, and how stable IQ rankings are from childhood to adulthood.
IQ performance by age: what the research shows
The table below synthesises data from the major longitudinal and cross-sectional studies of cognitive ability across the lifespan, including the Seattle Longitudinal Study (Schaie, 1994), the Hartshorne & Germine (2015) large-scale online study, and WAIS normative data. It shows the approximate relative performance on key cognitive dimensions at each age decade.
| Age range | Fluid reasoning / Novel problem-solving | Processing speed | Working memory | Vocabulary / Crystallised knowledge | Overall IQ test trend | Stability vs adult IQ |
|---|---|---|---|---|---|---|
| 5–9 yrs | Rapidly growing | Growing | Growing | Building rapidly | Rising fast | r ≈ 0.4–0.5 |
| 10–14 yrs | Growing strongly | Still growing | Still growing | Expanding fast | Rising | r ≈ 0.65–0.75 |
| 15–19 yrs | Near peak | Approaching peak | Near peak | Still growing | Approaching peak | r ≈ 0.75–0.85 |
| 20–29 yrs | Peak | Peak | Peak | Still growing | Peak overall | Baseline |
| 30–39 yrs | Slight decline | Slight decline | Slight decline | Growing | Very stable | r ≈ 0.85–0.90 |
| 40–49 yrs | Gradual decline | Gradual decline | Gradual decline | Near peak / plateau | Largely stable | r ≈ 0.85–0.90 |
| 50–59 yrs | Noticeable decline | Noticeable decline | Moderate decline | Peak or plateau | Slow decline | r ≈ 0.80–0.88 |
| 60–69 yrs | Significant decline | Significant decline | Moderate–significant decline | Stable or slight decline | Moderate decline | r ≈ 0.72–0.82 |
| 70–79 yrs | Clear decline | Clear decline | Clear decline | Gradual decline begins | Notable decline | r ≈ 0.60–0.72 |
| 80+ yrs | Substantial decline | Substantial decline | Substantial decline | Gradual decline | Clear decline | r ≈ 0.50–0.65 |
The two types of intelligence that diverge with age
Understanding the age trajectory requires understanding the distinction between fluid and crystallised intelligence — the most important split in the science of cognitive ageing.
Fluid intelligence (Gf) is the capacity to solve novel problems without relying on prior knowledge — pattern recognition, abstract reasoning, novel inference. Think matrix puzzles, sequences, working memory tasks. Fluid intelligence is largely independent of what you have learned. It peaks in the early-to-mid 20s and declines gradually but consistently across adulthood.
Crystallised intelligence (Gc) is the accumulated knowledge, vocabulary, and procedures that you have learned over a lifetime. Verbal comprehension, factual recall, and domain expertise are all expressions of Gc. Unlike fluid intelligence, crystallised intelligence continues growing throughout the 30s, 40s, 50s, and often into the 60s before eventually levelling and declining. A 60-year-old expert will typically far outperform a 25-year-old on domain knowledge and vocabulary despite lower fluid reasoning scores.
| Dimension | Peaks at | Trajectory in 30s–50s | Trajectory in 60s+ | Main influencing factors |
|---|---|---|---|---|
| Fluid intelligence (Gf) Pattern recognition, novel reasoning |
Early-to-mid 20s | Slow, gradual decline (~0.5–1 point/decade on fluid tasks) | Accelerating decline; most susceptible to aging | Processing speed, working memory capacity, neurological health |
| Crystallised intelligence (Gc) Vocabulary, knowledge, expertise |
50s–60s (broad peak) | Continues growing; most adults improve through 50s | Slow, gradual decline in 70s–80s | Education level, reading habit, intellectual engagement, career complexity |
| Processing speed How quickly you process information |
Late teens to early 20s | Steady decline from 25 onward; most salient change in aging | Clear, consistent decline | Neurological health, cardiovascular fitness, sleep quality |
| Working memory Holding and manipulating information |
Early-to-mid 20s | Gradual decline; more pronounced under time pressure | Significant decline, especially for complex tasks | Attentional control, sleep, exercise, cognitive engagement |
| Verbal comprehension Understanding and using language |
50s–70s | Growing or stable; peaks later than other abilities | Stable or slow decline; most preserved in aging | Reading, education, social engagement, intellectual challenge |
IQ stability: how reliable are childhood scores?
One of the most practically important questions about IQ across the lifespan is how stable scores are — and the answer is much more nuanced than popular accounts suggest.
The key finding, replicated across dozens of longitudinal studies, is that IQ stability increases with age. The correlation between a score at age 5 and adult IQ (age 18) is approximately r = 0.4–0.5 — moderately predictive but far from fixed. By age 10–12, the correlation to adult IQ rises to approximately r = 0.65–0.75. By adolescence (ages 15–18), it reaches r = 0.75–0.85. In adulthood, IQ rankings are relatively stable across decades, though the absolute performance on fluid tasks declines.
The Dunedin Multidisciplinary Health and Development Study — one of the most comprehensive longitudinal studies ever conducted — followed participants from birth to age 38. It found that IQ at age 7 predicted IQ at age 38 with a correlation of approximately r = 0.67, confirming meaningful but imperfect stability from childhood to adulthood.
This means a childhood IQ score is informative but not deterministic. A score in the average range at age 8 does not preclude significant cognitive development. Educational quality, nutrition, health events, and the level of cognitive challenge in a person's environment all continue to shape cognitive ability throughout childhood and adolescence.
What actually slows down first
Speed is the first casualty. Reaction times begin slowing measurably from the mid-20s — not dramatically, but consistently. This affects timed cognitive tasks disproportionately. Most observed "cognitive decline" in aging adults is heavily confounded with slower processing speed. When untimed cognitive tasks are used, age differences shrink substantially.
Working memory narrows. The ability to hold multiple pieces of information active simultaneously — crucial for complex reasoning and mental arithmetic — declines gradually from around age 30. This is why older adults often struggle more with multitasking and following complex multi-step instructions than with simple familiar tasks.
Attentional control weakens. Older adults are more susceptible to distraction than younger adults and require more cognitive effort to suppress irrelevant information. In a quiet environment with no distractions, the performance gap between younger and older adults on many cognitive tasks narrows considerably.
What preserves cognitive ability into later life
The most important finding from cognitive ageing research is that the rate of decline is not fixed — it is substantially modifiable. Several factors are robustly associated with slower cognitive decline:
Education. Higher educational attainment is consistently associated with later onset and slower progression of cognitive decline. The leading explanation is cognitive reserve — higher educated brains have built more redundant neural pathways, providing a buffer against age-related loss. The benefit is not just from formal education; cognitively demanding careers provide similar protection.
Cardiovascular health and aerobic exercise. The brain is heavily dependent on blood supply. Hypertension, diabetes, and cardiovascular disease are among the strongest risk factors for accelerated cognitive decline. Regular aerobic exercise — which improves cerebrovascular health and increases BDNF — is one of the most evidence-based interventions for preserving cognitive function with age. The research by Hillman, Erickson, and Kramer (2008) and subsequent meta-analyses place effect sizes for aerobic exercise on executive function at d = 0.3–0.6 in older adults.
Sleep quality. Chronic poor sleep accelerates cognitive decline through multiple mechanisms: impaired memory consolidation, reduced glymphatic clearance of neurotoxic waste products (including amyloid-beta, implicated in Alzheimer's), and elevated neuroinflammation. Sleep is not a passive state — it is the brain's primary maintenance window.
Social engagement. Social isolation is associated with accelerated cognitive decline; rich social networks and regular meaningful social interaction appear protective. The mechanism likely involves the cognitive demands of complex social interaction and the neurological effects of social connection on stress systems.
None of these interventions prevent decline entirely. But the difference between an 80-year-old who has been cognitively active, physically fit, and socially engaged versus one who has not is substantial — potentially many years of meaningful cognitive function.
For a deeper understanding of fluid vs crystallised intelligence and how to think about each, see our article on Fluid vs Crystallized Intelligence. For how to keep cognitive performance as high as possible throughout life, see How to Increase Your IQ. And if you want to measure where you currently stand, the AurorIQ test is free and uses IRT scoring — no email required.
Frequently asked questions
What is the average IQ for a 12-year-old?
An IQ of 100 is exactly average for a 12-year-old — just as it is average for any age, because IQ tests are normed to the specific age group. A 12-year-old scoring 100 is performing at the level of the typical 12-year-old. Scores of 90–110 are the normal range, 110–120 are above average, 120–130 are superior, and above 130 is very superior or gifted. These thresholds are identical at every age — only the reference group changes.
At what age does IQ peak?
It depends which component. Fluid intelligence — novel problem-solving, abstract reasoning, processing speed — peaks in the early-to-mid 20s (roughly 18–25). Crystallised intelligence — vocabulary, accumulated knowledge, expertise — continues growing through the 40s and 50s, sometimes into the 60s. On standardised IQ tests (which emphasise fluid reasoning), overall scores peak in the mid-20s. But on knowledge-heavy tests, older adults often outperform younger ones.
Does IQ decrease with age?
Selectively. Processing speed, working memory, and novel abstract reasoning show gradual declines from the late 20s onward, accelerating after 60. Vocabulary and factual knowledge remain stable or continue growing into later adulthood. Population-level IQ test scores decline from the 60s onward, but the decline is slower than commonly assumed. Cognitive reserve from education, physical fitness, and intellectual engagement substantially slows the rate of decline.
Is IQ stable throughout life?
It becomes more stable with age. Before age 5, scores have low predictive validity for adult IQ. From age 10–12, the correlation with age-18 IQ is approximately r = 0.65–0.75. By adolescence it reaches r = 0.75–0.85. In adulthood, IQ rankings are relatively stable across decades. A childhood score — especially before age 8 — should be treated as informative but not fixed. Significant changes in educational quality, nutrition, or health environment can meaningfully shift scores.
What is a good IQ for a 10-year-old?
A score of 100 is exactly average for a 10-year-old. Scores of 90–110 are the average range and are entirely normal. Scores of 110–120 are high average (top 25%), 120–130 are superior (top 9%), and above 130 is very superior or gifted (top 2.3%). Because IQ is always normed to age, a "good" score at age 10 means the same thing as a "good" score at any age — it just measures performance relative to other 10-year-olds.
Does intelligence decline in old age?
Yes, selectively and more slowly than popular belief suggests. Processing speed, working memory, and fluid reasoning show consistent declines from the 30s onward, accelerating after 60. Verbal knowledge and crystallised intelligence remain relatively stable or keep growing into the 70s. The rate of decline is substantially modifiable: physical health, aerobic exercise, social engagement, sleep quality, and continued intellectual challenge are all associated with significantly slower cognitive decline in later life.