There is no shortage of anecdotes and conjectures on both sides of this widespread belief, but good supporting data either way is harder to find. Can anyone provide any references for serious (preferably academic rather than journalistic) research that actually crunched the data and produced interesting conclusions about whether this bit of folklore is reality-based?
I put "mathematicians do their best work when they're young" in quotes because this is clearly not a well-posed question–it is only intended to be shorthand for any of a number of questions on this topic.
Historical studies (Évariste Galois, etc.) are OK, but studies on people born after, say, 1950, would be of greater interest and relevance.
Best Answer
These two studies arrive at what seems to be a more sensible conclusion:
Age and Scientific Performance, Stephen Cole (1976).
Age and Achievement in Mathematics: A Case-Study in the Sociology of Science, Nancy Stern (1978).
For anecdotes and "advice to aging mathematicians", I might recommend Mathematical menopause, or, a young man's game?, by Reuben Hersh (The Mathematical Intelligencer, 2001).