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Average IQ is 100 at twenty, at fifty and at eighty — not because nothing changes, but because the test re-scores you against your own age group every time. Drag through a lifetime and watch the two numbers separate.
One lifetime, three trajectories
The flat line is your reported IQ. It is flat by construction, not by luck. Drag the scrubber, or click anywhere on the chart.
| Age | Reported IQ | Fluid reasoning | Crystallised knowledge |
|---|---|---|---|
| 20 | 100 | 100 (±0) | 100 (±0) |
| 30 | 100 | 100 (±0) | 102.1 (+2.1) |
| 40 | 100 | 97 (−3) | 104.1 (+4.1) |
| 50 | 100 | 94 (−6) | 106.2 (+6.2) |
| 60 | 100 | 91 (−9) | 108.3 (+8.3) |
| 70 | 100 | 88 (−12) | 106.8 (+6.8) |
| 80 | 100 | 85 (−15) | 105.3 (+5.3) |
| 90 | 100 | 82 (−18) | 103.8 (+3.8) |
At 40, a test would still report your IQ as 100 — because it is scored against other 40-year-olds. Measured against a 25-year-old instead, the reasoning half of that score sits nearer 97, while the knowledge half sits nearer 104.1.
These are population-average trajectories, not a prediction about you. Individual variation is far larger than the lines above, and health, education and practice all move them. See the confidence range on a single score.
Your age changes the norm group — never the ruler. Every score on this page is already measured against people your own age. What it cannot tell you is whether the test that produced it was age-normed at all. If yours came from a quiz that applied one answer key to everyone, the age curve above is describing someone else.
If you have seen a table claiming “average IQ at 45 is 106”, it was measuring something else and calling it IQ. Here is what a test actually does with your age.
You get a raw score — how many items you solved, and how quickly. This number really does change across a lifetime, and on its own it means nothing.
The raw score is looked up in a table built from people of roughly your age. A 70-year-old is never scored against 25-year-olds. This is the step almost every “IQ by age” chart ignores.
Each age band is re-centred on 100 with a standard deviation of 15. So the average is 100 at 20 and at 80 by definition. A flat line is not a discovery; it is the arithmetic working correctly.
If the averages differ between age rows, one of three things is true: they are raw scores mislabelled as IQ, they came from an instrument that was never age-normed, or they were invented. There is no fourth option. A properly normed test cannot produce a non-100 average for its own norming group — that is what norming means.
“Intelligence” is not one quantity that rises or falls together. The part that solves novel problems and the part that stores what you already know follow almost opposite paths — which is why a single number can stay flat while both halves move.
Solving problems you have never seen: matrix puzzles, number series, mental rotation, processing speed. It peaks in the mid-twenties and falls from roughly age 30 with no plateau — the decline does not pause and then resume, it is steady.
What you have accumulated: vocabulary, general knowledge, professional judgement, verbal comprehension. It keeps improving for decades and typically peaks somewhere around age 60 before drifting down slowly.
| Age band | Average reported IQ | Fluid reasoning vs a 25-year-old | Crystallised knowledge vs a 25-year-old |
|---|---|---|---|
| 20–29 | 100 | 100 | 100–102 |
| 30–39 | 100 | 100 to 97 | 102–104 |
| 40–49 | 100 | 97 to 94 | 104–106 |
| 50–59 | 100 | 94 to 91 | 106–108 |
| 60–69 | 100 | 91 to 88 | 108 peak |
| 70–79 | 100 | 88 to 85 | 107–105 |
| 80+ | 100 | 85 and below | 105–104 |
Where these numbers come from, and how far to trust them. The trajectories follow the pattern reported by Salthouse and by the WAIS-IV normative sample (N = 1,577), where the correlation between age and reasoning performance runs at roughly r = −0.40. The honest caveat is that the two ways of measuring this disagree in opposite directions: cross-sectional studies compare different people born decades apart and so overstate individual decline (a cohort confound), while longitudinal studies retest the same people and understate it, because practice effects inflate later scores and the least healthy participants drop out. The shaded band on the chart above is that disagreement drawn honestly — roughly −0.015 to −0.025 SD per year — rather than a single confident line.
Age-norming matters most for children, because a year of development at six is worth far more than a year at forty-six. It is also why a single early score should never be treated as a verdict.
Children are normed in narrow bands — often three or four months wide — because development is so fast at that age. The same raw performance from a child who is six years two months and one who is six years eleven months produces meaningfully different IQ scores.
The younger the child, the weaker the link between today’s score and their adult score. Scores measured in the early school years move substantially by adolescence; by the teenage years they settle considerably. Treat an early result as a snapshot of current performance, not a ceiling.
The original ratio IQ was mental age ÷ chronological age × 100. It worked passably for children and broke completely for adults — it implies ability keeps rising with age forever, which is why every modern test abandoned it for the age-normed deviation score used today.
Raw ability moves across a lifetime — some of it climbs into your fifties, some of it slips from your twenties. That is precisely why a score is never a raw count: it is always your position among people born around the same time as you. A number quoted without the age band it was normed against is not wrong so much as unfinished.
Normed within age band, scored on the standard scale — mean 100, standard deviation 15.
Your reported IQ score does not systematically change with age, because IQ tests are age-normed — each age band is re-centred so that its average is 100. What does change is raw performance. Fluid reasoning (novel problem solving) declines from about age 30 at roughly 3 IQ points per decade measured against a young-adult reference, while crystallised knowledge (vocabulary, facts, judgement) rises into your sixties. A single IQ number can stay flat while both of those move. IQ over time shows how the norms themselves shift beneath that number.
100 at every age. This is true by construction rather than by coincidence: test publishers build separate norm tables for each age band and set the mean of each band to 100 with a standard deviation of 15. Any table showing different averages for different ages is reporting raw scores, using a test that was never age-normed, or is fabricated. The IQ percentile calculator shows what any given average actually corresponds to.
There is no single peak, because the underlying abilities peak at different times. Fluid reasoning peaks in the mid-twenties. Processing speed peaks earlier still. Crystallised knowledge peaks around age 60. Vocabulary can keep improving even later. The idea of one peak age comes from treating intelligence as a single quantity, which the evidence does not support. The IQ bell curve shows how the population spreads out at every level.
Not the number a test reports, no. If you take a properly normed test at 65, you are compared with other 65-year-olds, so a stable relative standing produces a stable score. What declines is your performance relative to a younger reference group. Both statements are true at once, and confusing them is the single most common error in this topic.
Most free online tests are not age-normed at all — they compare everyone against a single undifferentiated table, often weighted toward younger test-takers, and many are timed in ways that penalise slower processing speed rather than weaker reasoning. That produces a lower number without measuring lower ability. It is worth checking whether a test states its norming approach before reading anything into the result. Our page on how accurate IQ tests are covers what one sitting can and cannot establish.
Less accurate than the single number suggests. Every score carries a margin of error, and a typical online test produces a 95% confidence range of roughly ±9 points. That range is usually wider than the differences people argue about. You can see this directly on the percentile calculator, or check whether a gap between two scores is real on the comparison tool.
They separate sharply. Processing speed and working memory — the fluid side — peak early and drift down from the twenties onward. Vocabulary, general knowledge and judgement — the crystallised side — often keep climbing into the sixties. A composite IQ can sit perfectly still while both halves move in opposite directions underneath it. IQ over time charts that split, and the IQ bell curve shows how each level is distributed.
With your own age group, because that is what the score already means. IQ is not an absolute quantity; it is your standing among people normed alongside you. Comparing a 70-year-old’s score against 25-year-olds is not a harder test, it is a different question entirely. The IQ percentile calculator gives the exact standing for any score.
Related, but pulled toward the average. Children of high-scoring parents tend to score above the mean and below their parents — not because inheritance is weak, but because any single measurement carries luck that does not pass on. Regression to the mean explains the arithmetic.
An age-normed score, its percentile and its confidence range — stated up front, not buried. That is the only version of this number worth having.
