A person’s IQ is a measure of how they perform on an IQ test relative to others of the same age. Consequently, as a person ages, their performance on the IQ test relative to others of the same age does not change significantly. IQ scores are adjusted to account for your age, so your IQ relative to others the same age, does not change appreciably through your adult years.
However, after you reach your mid-to-late 20’s, your fluid intelligence, as measured by IQ tests like the Raven’s Progressive Matrices, decreases. A direct assessment of the trajectory of human intelligence throughout a lifetime came in 1936, when John Raven developed his Raven’s Progressive Matrices tests of intelligence. To norm these tests, i.e., establish baseline data on what scores adults could be expected to achieve as they age, he conducted widespread testing in the United Kingdom, Ireland, and Canada between 1939 and 1947. His graph of the Standard Progressive Matrix (SPM) scores for people of different ability levels between ages 20 and 65 is reproduced in the graph below (from Intelligence, IQ & Perception).
Note: A person who scores at the 95th percentile of a particular age cohort has scored higher than 95% of all the people of that age. To be at the 95th percentile of the 20-year-old age cohort requires an SPM score of 55, while being at the 95th percentile of the 50-year-old age cohort requires only a score of 48.
The 50th percentile line in the graph shows that the average SPM score for 20-year-olds was about 43, while the average SPM score for 50-year-olds was about 33. This shows a substantial loss of cognition between a person of age 20 and a person of age 50, suggesting that intelligence is declining during that period.
The graph shows that the fluid intelligence of people declines as they age. However, the people of higher IQ decline at a slower rate than those of lower IQ. In abstract fields like pure mathematics and theoretical physics the breakthroughs are achieved by people between early adulthood and early middle age. After that, people like Stephen Hawking can make discoveries, but their best and most creative work is usually behind them. More is available at: At What age Does one pass his/her intellectual peak? – Intelligence and IQ
The good news, however, is that our intelligence has two essential components: fluid intelligence (mental efficiency or acuity) and crystallized intelligence (learning from experiences). As our fluid intelligence decreases, our accumulated experience brings an increase in our crystallized intelligence. That’s why the average age of people appointed to the U.S. Supreme Court is 50 years. Judgement and perspective are acquired through experience.
The main advantage of education, whether formal or informal, is that it stimulates learning and the development of crystallized intelligence, the way learning a skill enhances performance in that skill. A great athlete or musician may be at the peak of their physical or creative powers in their early 20’s, but training and practice are required to bring them to peak performance a decade or two later.
