One theory is the it’s because it takes a while to absorb the knowledge of your predecessors.
Scientists spend ages 5 through 18 in school, and then ages 18 through 30ish getting their academic degrees. Then a few years of learning on the job, and presto! . Meanwhile, scientific breakthroughs tend to be less common in old age because we invest less in learning as we get older, and our skills gradually become less relevant.
Personally, I’m not sure I buy the explanation of the drop-off.
And the age continued to get older over the 20th century.
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