Average IQ by Age: Why a 10-Year-Old and a 60-Year-Old Both Average 100

Every age group averages 100

Deviation IQ is a comparison with peers. Britannica's account of test norms puts it precisely: the deviation IQ has a mean of 100 "for each age level." A child's raw answers are converted using a table for children of that age; an adult's, using a table for adults of that age band.

The major tests are built this way. The Wechsler Intelligence Scale for Children covers ages 6 to 16. The Wechsler Adult Intelligence Scale, fourth edition, was standardized in 2008 on 2,200 Americans aged 16 to 90. In each case the middle of every age group is set at 100, and 15 points mark one standard deviation.

So the honest answer to "average IQ by age" is a flat line at 100, laid over curves that rise and fall. For the anchor itself, see what is the average IQ and IQ 100.

Why the original IQ broke down for adults

The first IQ was a ratio. The German psychologist William Stern coined the term in 1912 and proposed dividing a child's mental age by chronological age. Lewis Terman, revising the Binet test at Stanford in 1916, multiplied the quotient by 100. A 10-year-old performing like a typical 12-year-old scored 120.

The formula works only while ability grows with age. Adults stop gaining on most tests while their birthdays keep coming, so the ratio would fall every year for no reason. Early testers patched the problem by freezing age: Britannica notes that under the ratio method, "the chronological age of any individual older than 18 is taken as 18." David Wechsler abandoned mental age entirely with the Wechsler-Bellevue scale of 1939, judging that mental age norms did not apply to adults. His point-scale approach, scored against same-age groups, became the standard. The background is in the history of the IQ test.

Fluid and crystallized: two curves, not one

Raymond Cattell split intelligence into two broad parts in 1943, and he and his former student John Horn tested the split in a 1967 paper in Acta Psychologica titled "Age differences in fluid and crystallized intelligence."

Fluid intelligence is reasoning on new problems with little help from prior learning: spotting the rule in an unfamiliar pattern, holding several steps in mind. Crystallized intelligence is accumulated knowledge and skill, such as vocabulary and general information. Textbook summaries describe fluid ability peaking in the 20s and declining gradually afterward (Cacioppo, 2013), while crystallized ability holds steady through most of adult life and declines only after about 65 (Cavanaugh and Blanchard-Fields, 2006).

A full-scale IQ blends both. That is one reason a 70-year-old's raw vocabulary can beat a 25-year-old's while the same person's timed puzzle scores lag. Matrix tests such as Raven's Progressive Matrices lean heavily on the fluid side.

When decline starts: the Salthouse study

Timothy Salthouse of the University of Virginia asked the blunt question in 2009, in Neurobiology of Aging: when does age-related cognitive decline begin? His answer, from 2,350 healthy, well-educated adults aged 18 to 60, was early. The data, he wrote, indicate that "some aspects of age-related cognitive decline begin in healthy educated adults when they are in their 20s and 30s."

Across 12 tests of reasoning, spatial visualization, episodic memory and perceptual speed, peak performance fell between ages 22 and 27. From 18 to 60, speed and spatial scores dropped by about one standard deviation, and reasoning and memory by about 0.6 to 0.7. Vocabulary and general knowledge moved the other way, rising until at least 60.

Salthouse also explained why some long-term studies show no early decline. People retested on the same tasks improve from practice, and those retest gains, which he measured three ways, were generally larger than the age differences. Subtract the practice effect and the longitudinal picture looks much like the cross-sectional one.

No single peak

Joshua Hartshorne of MIT and Laura Germine of Massachusetts General Hospital mapped the curves in finer detail in Psychological Science in 2015. They combined archival Wechsler data with online tests taken by nearly three million people, then ran a further experiment with almost 50,000 participants.

Each ability kept its own schedule. Raw processing speed peaked around 18 or 19 and fell right away. Short-term memory improved until about 25, held, and began dropping around 35. The ability to judge other people's emotional states peaked in the 40s or 50s. Vocabulary, which the older Wechsler data placed in the late 40s, peaked in the late 60s or early 70s in the new data. The authors concluded there is no single age at which a person is sharpest across the board.

Your rank is more stable than your raw score

Raw ability moves; relative standing moves far less. The clearest evidence comes from Scotland. On 1 June 1932, the Scottish Mental Survey gave the Moray House Test to almost every child born in 1921, 87,498 children in all. Ian Deary, Alison Pattie and John Starr retested 106 members of the Lothian Birth Cohort of 1921 on the same test at age 90. The correlation between their scores 79 years apart was .54, or .67 after correcting for range restriction, as reported in Psychological Science in 2013.

That is a strong link over almost eight decades, and far from perfect. People do change places. Health, education and the Flynn effect, the long rise in raw scores across generations, all shape where someone ends up. That last factor also distorts snapshots that compare today's 70-year-olds with today's 20-year-olds, since the two groups grew up in different eras.

Reading scores across ages

Because every age is normed separately, an IQ score tells you about rank among peers, not about raw capability. A 115 at age 9 and a 115 at age 59 both mean "higher than about 84 percent of people that age." They do not mean the two people can solve the same problems.

The test edition matters just as much. Children are assessed on scales such as the WISC-V, adults on the WAIS-IV or its 2024 successor; see the WISC-V explained and the WAIS-IV explained. Online quizzes, including our free IQ test, give a rough practice reading at best, as are online IQ tests accurate explains. For a child, scores depend on the test, the day and the examiner's judgment, and a licensed psychologist interprets these scores. More context sits in our IQ guides.

Editorial standard: IQ figures for historical figures are retrospective estimates and are labeled as such; scores for living people appear only when the person disclosed them or a reputable outlet reported them. Corrections: contact the editors.