Maybe not in eir early- or mid- 20s, since nowadays at that age it is hard to even have enough familiarity to be able to define the major open problems (most of

Maybe not in eir early- or mid- 20s, since nowadays at that age it is hard to even have enough familiarity to be able to define the major open problems (most of my colleagues focus a lot on devouring book to try to get ahead of this curve). Still, major work is generally done by one in eir late 20s or early 30s. It has long been the case, and still continues to be (Green and Tao proved their famous theorem at the age of 29). Hardy lamented that math is a young person's game. The book "A Beautiful Mind" discusses how Nash's 30th birthday was a gloomy one, since it generally means the end of one's major contributions to the field. You can't even win the Fields medal unless you're under 40. Now, of course there will be counter-examples, but these are not the majority, and generally come in those fields where you have to digest a great amount of material and push your way through for long stretches of time (grad students and post-docs also tend to figure prominently in these cases)