"... It is true — certainly in mathematics; what I say now will not apply to the School of Historical Studies where people tend to make their main contributions at a much later age — in mathematics and physics, the prime period in one's life in probably over by, say, 45 or so. That's a bit conservative. Some might put it a bit earlier. Of course one can continue to work, and do very good work afterwards, but certainly the most productive period would be before that, between 25 and 45."