I am going to have to say:
Poker has become "popular". People that are.... At a "higher level of maturity" are less likely to go along with trends, find new hobbies and they are definitely less likely to go throw away a bunch of money
gambling.
A MAJOR FACTOR is also that, poker is a form of gambling. At some point, everyone has a loosing streak. Never understood why my winning streaks never even last 1/4 of the time of my loosing streaks do myself.
Most people go broke or quit before they go broke at some point. A few actually make money and even less can make a living. It takes a smart gambler to win. So it is reasonable to assume a smart gambler would quit after winning enough to live off of.
I guess what i am saying is that.... As you get older there are more and more reasons not to play. Therefor the majority of players are younger, thus more younger players will win.
Its also worth mentioning that media coverage of any sport will only give so much attention to 1 given player. There's not too much excitement about 1 player winning a
poker tournament constantly. Poker hardly ever has any real drama to attract fans. So they are going to focus on the new and upcoming players. No one gets excited about what place
Doyle Brunson finishes in. If you get my point?
To answer your original question, I know of no studies, books, or discussions of this subject any place other than rite here.

If you do find any let us know.