When do players reach their prime?

And what kind of research has been done on answering this question?

I’m answering this off the top of my head, so nobody gripe at me for not sourcing properly:

The generally accepted peak year is the Age-27 season.  James, IIRC, did some research and found that it might be a year or two behind that.  You’ll still find mainstream (for stats people) acceptance behind 27, but somewhere between 27 and 29 is going to hit 95% of observers.  If you’re looking for research, one of James’ Abstracts (hand to God, I don’t remember which) is the way to go.

Thanks. Sorry for the delay in replying.