Average IQ by Age: What the Data Actually Shows.
Searches for "average IQ by age" usually expect a table where the number climbs through childhood, peaks, and then falls. That table does not exist, and the reason is built into how the score is defined. IQ is a relative measure: it places you against other people of your own age, so the age-group average is fixed at about 100 by design. A 20-year-old and a 70-year-old who each score 100 are both exactly average for their age, even though their raw cognitive performance differs.
Average IQ by age
Because IQ scores are age-normed by construction - each person is compared against their own age group - the average IQ is set to about 100 at every age, from childhood through old age. So "average IQ by age" is not a rising or falling curve. The meaningful question is how raw cognitive abilities change over a lifetime: speed-based and novel-reasoning skills tend to peak in the late teens to late twenties and decline gradually after, while knowledge-based abilities such as vocabulary stay stable or keep rising into middle age.
That does not mean cognition is unchanging across life. It changes a great deal, just not in a way the IQ scale is built to show. This page separates the two ideas the question tangles together: the age-adjusted score (always centered on 100) and the underlying raw abilities (which follow real, well-documented trajectories).
Why the average is 100 at every age
IQ norming works by collecting performance from a large, representative sample within each age band and then rescaling so that the average for that band is 100 and the standard deviation is 15. A child is scored against other children; an adult against other adults of similar age. This is what makes the number interpretable at all - "100" means "right in the middle of your peers."
A direct consequence is that you cannot read aging off the IQ scale. If everyone in an age group slows at the same rate, the rescaling absorbs the change and the average stays at 100. The scale is deliberately blind to absolute, cross-age differences in raw ability, which is exactly why a separate body of research is needed to describe how cognition actually moves over the lifespan.
What does change: fluid abilities peak early, then decline
Fluid abilities - reasoning with novel problems, holding and manipulating information, and raw processing speed - tend to reach their peak in the late teens to late twenties and then decline gradually across adulthood. This is the part of cognition that powers solving an unfamiliar puzzle quickly or spotting the pattern in a matrix you have never seen.
Large studies of cognitive aging find that some of these speed-based and reasoning measures begin a slow, measurable decline earlier in adulthood than people expect, though the everyday effect is small for decades and varies widely between individuals. Group averages describe a population trend; they do not forecast any single person, and many older adults outperform much younger ones on the same tasks.
What holds up: crystallized knowledge stays stable or rises
Crystallized abilities - vocabulary, general knowledge, and the accumulated facts and verbal skill you have built over a lifetime - follow a very different curve. They typically stay stable or keep rising into middle age and are well preserved into later life, declining only modestly and late.
This split is why a precise summary of "how smart someone is at a given age" misleads. A 60-year-old may reason through a brand-new abstract puzzle a little more slowly than a 25-year-old while bringing far richer knowledge, vocabulary, and judgment to a familiar problem. Two different abilities, two different age trajectories, and a single IQ point hides both.
- Fluid reasoning and processing speed: peak in late teens to late twenties, then gradual decline
- Working memory: similar early peak, slow decline through adulthood
- Vocabulary and general knowledge: stable or rising into middle age, well preserved later
- Individual variation is large at every age - averages are not predictions about you
What this means for an online score
A reputable online test that reports an age-adjusted IQ is already comparing you against your own age group, so your result is not inflated or deflated by how old you are. A 100 means average for your age, whether you are 18 or 68.
Where age does matter is in interpreting the parts. A domain breakdown that leans on speed and novel reasoning may read a little lower with age while a knowledge-based verbal domain may read a little higher - a normal pattern, not a deficit. As with any unsupervised test, read the result as a band rather than a precise point, and treat it as one snapshot of reasoning performance, not a verdict on your mind.
Also relevant: See a sample IQ report, Read the IQ test guide
Frequently asked questions
Does IQ decrease with age?
The age-adjusted IQ score does not systematically fall, because everyone is compared against their own age group, so the average stays near 100 at every age. Underlying raw abilities do change: fluid reasoning and processing speed decline gradually through adulthood, while crystallized knowledge such as vocabulary holds steady or rises into middle age.
At what age is IQ highest?
There is no single peak age for IQ itself, because the score is normed to average 100 at every age. Raw fluid abilities - novel reasoning and processing speed - tend to peak in the late teens to late twenties, while crystallized knowledge keeps building into middle age, so different abilities peak at different times.
What is the average IQ for adults?
About 100, by definition. The IQ scale is built so that the average of the reference population is 100 with a standard deviation of 15, and this holds within each adult age group because scoring is age-normed.
Is a childhood IQ score permanent?
Not exactly. IQ becomes more stable through adolescence into adulthood, but a single childhood score has measurement error and can shift, especially when taken young or under poor testing conditions. It is best read as an estimate, not a fixed lifelong figure.
Does the Psychology.me IQ test adjust for age?
No. Professionally administered tests like the WAIS and WISC use separate age-band norms, which is what makes the average 100 at every age. Our online General IQ test is normed against an adult population rather than age bands, so it gives a valid adult-referenced estimate but is not age-adjusted within adulthood. For a precise, age-normed score, an individually supervised test is the right tool.
References
- Salthouse, T. A. (2009). When does age-related cognitive decline begin?. Neurobiology of Aging, 30(4), 507-514.
- Cattell, R. B. (1963). Theory of fluid and crystallized intelligence: A critical experiment. Journal of Educational Psychology, 54(1), 1-22.
- Horn, J. L., & Cattell, R. B. (1967). Age differences in fluid and crystallized intelligence. Acta Psychologica, 26, 107-129.
- Deary, I. J. (2012). Intelligence. Annual Review of Psychology, 63, 453-482.
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