The average IQ is 100 at every age — from 8 to 80 — because IQ tests are age-normed, meaning your score compares you only to people your own age. This is the single most common misunderstanding about IQ and age. There is no table where 20-somethings average 107 and retirees average 93; any such table misreads how the scoring works. What does change across life is the raw cognitive machinery underneath the score: how fast you process information, how much you can hold in mind, and how much knowledge you have accumulated. This article explains both layers — the score that stays put, and the abilities that move.
Why the average is 100 at every age
When a test like the WAIS is standardized, the norming sample is split into age bands, and each band gets its own conversion from raw performance to IQ. A 72-year-old's answers are compared to other people in their late 60s and 70s — never to 25-year-olds. The average member of every age band therefore lands at exactly 100, by construction. (The same logic pins the overall average IQ to 100 for the whole population.)
A useful consequence: your IQ score tends to be fairly stable across adulthood even though your raw abilities are changing, because your age-mates are changing in roughly the same way. If your processing speed slows at the typical rate, your rank within your cohort — and therefore your IQ — barely moves.
How raw cognitive abilities change by decade
Underneath the age-norming, large studies that test people across the lifespan — including Hartshorne and Germine's analysis of tens of thousands of test-takers — show a consistent broad pattern, with different abilities peaking at strikingly different ages:
| Decade | Typical pattern in raw abilities |
|---|---|
| 20s | Fluid reasoning, processing speed and working memory at or near lifetime peak; knowledge base still growing |
| 30s–40s | Crystallized knowledge rising steadily; fluid abilities begin a slow, gradual decline that is rarely noticeable day to day |
| 50s–60s | Vocabulary and general knowledge reach their lifetime peak — often in the 60s; fluid decline continues gently |
| 70s and beyond | Broader decline across most raw abilities, but with very large individual variation — many people stay sharp well into their 80s |
Two honest caveats. First, much of this evidence is cross-sectional (comparing today's 30-year-olds with today's 70-year-olds), which can exaggerate decline because older cohorts grew up with less schooling. Longitudinal studies that follow the same people generally find later and gentler declines. Second, averages hide enormous spread — the variation within any age group dwarfs the difference between age groups.
Fluid vs. crystallized intelligence
The decade-by-decade pattern makes sense once you split intelligence the way psychologists Raymond Cattell and John Horn did:
- Fluid intelligenceis the ability to solve novel problems — spotting patterns, reasoning through puzzles you've never seen, holding several pieces of information in mind at once. It depends heavily on processing speed and working memory, peaks in the 20s, and declines gradually thereafter.
- Crystallized intelligence is accumulated knowledge and skill — vocabulary, facts, professional expertise, judgment built from experience. It keeps rising through midlife and often peaks in the 60s.
Real-world performance usually blends the two, which is why many careers — medicine, law, management, writing — see people doing their best work in their 40s and 50s: declining fluid horsepower is more than offset by a much larger knowledge base. IQ tests sample both kinds of ability, and the IQ scale reports the blended result relative to your age group.
What actually protects cognition with age
You cannot choose your birth year, but the research on healthy cognitive aging points to factors that are at least partly in your control:
- Education and mental engagement.More years of education and cognitively demanding work are associated with higher "cognitive reserve" — a buffer that delays the point at which decline becomes noticeable.
- Physical activity. Regular aerobic exercise is one of the most consistently supported protective factors, plausibly through better blood flow to the brain.
- Cardiovascular health. What harms the heart harms the brain: untreated hypertension, diabetes and smoking all accelerate cognitive aging.
- Sleep. Chronic short or fragmented sleep impairs memory consolidation and is linked to faster decline.
- Social connection. Isolation is a risk factor; staying socially engaged is protective.
Notice what's missing: commercial "brain training" apps, whose benefits mostly stay confined to the trained games. For a candid look at what does and doesn't move the needle, see how to increase your IQ.
Children and age-norms: how kids are scored
Age-norming matters even more in childhood, when abilities change month by month. Tests like the WISC (ages 6–16) and WPPSI (ages 2.5–7) compare each child to peers of the same age, often in bands just a few months wide. A 7-year-old who reasons like a typical 9-year-old will score well above 100 — not because they answered adult questions, but because they outperformed other 7-year-olds.
Historically this was literal: the original "intelligence quotient" divided mental age by chronological age and multiplied by 100. A 10-year-old performing at a 12-year-old level scored 120. Modern tests abandoned that formula for the deviation IQ — your distance from your age group's mean, expressed on a scale with SD 15 — because mental-age ratios break down entirely for adults. One practical caution for parents: scores before about age 6 are only weakly predictive of adult IQ, and stability increases through the school years. A single early test is a snapshot, not a destiny.
Does your IQ score itself stay stable across life?
Remarkably, yes — more than most people expect. The best evidence comes from rare studies that tested the same people decades apart. In the Scottish Mental Surveys, thousands of children tested at age 11 in 1932 and 1947 were retested in old age; the correlation between childhood and late-life scores was strong (roughly 0.6–0.7), meaning children who ranked high at 11 tended to rank high at 77. Rank order is not destiny, though: that correlation still leaves plenty of room for individuals to drift up or down, and the drift is exactly where lifestyle, health and education appear to matter most.
There is also a cohort effect worth knowing about. Because of the Flynn effect — the 20th-century rise in raw test performance of roughly three points per decade — each generation has tended to outperform the previous one on the sametest items. This is another reason cross-sectional comparisons overstate age-related decline: today's 75-year-olds are being compared with younger people who grew up in a more test-friendly, education-rich world. Age-norming quietly absorbs all of this, which is precisely why the published average stays pinned at 100 for every group.
The takeaway
"Average IQ by age" is a trick question: the average is 100 for every age group, because that is what age-norming means. The real story is the trade beneath the surface — fluid speed traded for crystallized depth as the decades pass — and the healthy habits that slow the losses. If you want to see where a score falls within your own age group, the IQ percentile table converts any score into a population rank. And remember that online tests give estimates, not clinical diagnoses — a formal assessment for school placement or medical purposes needs a professionally administered, age-normed test.