I've been thinking about this recently - particularly with the buzz surrounding Alex Pagulayan's attempt to make it as a snooker pro at the ripe old age of late 30s. Nice to see some stats on it.
I think it's a bit of an urban legend that it's a young man's game and that Ronnie O'Sullivan is an "exception" being able to crush the field heading towards his forties. In the 80s and 90s a new breed of young player came to the fore. They practised, trained, were coached, learned from their TV heroes - all like machines, and were technically and mentally streets ahead of the former generation at a similar age. Sport as a whole, and snooker in particular was going from being a bizarre mixture of a back room yet gentlemen's game to a fully fledged professional sport. Many of those youngsters were so good so young that they are still good enough to mix it on the level with the new young pros who are coming through. It's not that they have dropped off less quickly than the Davis' or even the Hendrys just that they were dropping from a slightly higher plateau but with skills that were ingrained much much deeper. Whether Ronnie, John Higgins and the rest can do a Ray Reardon is yet to be seen though.
In short, there was a huge step up at the top end of the game in the 80s and 90s (maybe led by Hendry who was the first of the wave - was he on the cusp and therefore also a part of the old wave or did he lose his hunger? I guess we'll see when he plays as a wild card this season) and then not much of a step up since, just more and more 20s at the top level than 30s - but will this plateau off too? And the young players learned early - without the need to endure years of experience (a bit like online poker players who have played 27 zegamegamillion hands at the age of 19).
All opinion, conjecture, streams of consciousness of course.