[mbeex]

G. H. Hardy wrote: "Mathematics is a young man's game." Of course, you can continue to be a mathematician later, but for top performance, especially in terms of novelty, you have to start early.

[defrost]

Euler started strong early and peaked later

    During his Berlin years (1741–1766, aged 34-59), Euler was at the peak of his productivity.

    He wrote 380 works, 275 of which were published.

~ https://en.wikipedia.org/wiki/Leonhard_Euler

Admittedly some of that was playing a Swiss Charles Dodgson to 15 and 10 year old girls.

~ https://en.wikipedia.org/wiki/Letters_to_a_German_Princess

[mbeex]

Individual examples do not contradict the general statement. Galois was dead at an age when I wasn't even at university. Abel a bit later (so avoid groups if you want to have a long life), same with Ramanujan (which incidentally may be a factor in Hardy's comment). And so on, just as singular at the first glance. As a mathematician, however, I continue to argue Hardy's point, both for the present and for the past as a general and observable phenomenon.

And the number of books as a measure of quality, really? I think that view is skewed by today's “publish and perish” environment (nothing against Euler).