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.
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: https://www.intelligence-and-iq.com/at-what-age/
