I've calculated the stat about the average age of the top-100 in 2012 comparing to 2011, and it still has increased by 1/2 year : 27.8 years old comparing to 27.3 (was 26.1 in 2007)
For those interested in the top-players as usual, the average age of the top-20 is the same as last year (26.9), as some oldies did drop out of the top-20 (Roddick, Fish, Lopez) and some youngsters arrived (Raonic, Nishikori, and Cilic came back).
If you look in detail, the main change of these last 5 years has rather been a lack of emergence of youngsters, rather than old players lasting longer (which is partly true but far less).
My opinion is that the main reason is that the game is more demanding physically and mentally. And the context reason which makes it worse for youngsters is that the level of the competition in challengers and futures has improved, which makes it even more demanding to make consistent results at this level.
There are also generational effects, especially for the top-players, but not only : it always strikes me how some calendar generations are so much better than others, even in-depth, not only for the top-players, for instance 1989 and 1993 generations are very poor, 1987 and 1981 were very good.
2) Prize money has stagnated below ATP level which means they are struggling in key years to play the most suitable schedule for them, hire coaches, physios, and the things like that which could start to give them an edge
in the old times they didn't even need physios and that sort of things
But well, yes, probably older players now have better methods to take care of their body ... and I would not be surprised at all if doping was one of them, and that older players know better about it than younger ones.
May be a matter of prize money but more a matter of experience imo, including the knowledge of one's own body.
Recently I've been surprised by how many youngsters get seriously injured and stay out for a long time (for instance Krajinovic and Carreno-Busta, even Christian Harrison when he was very young).
3) Points distribution was altered a couple of years ago which favoured later rounds over earlier rounds even more, and favoured ATP events even more over Challengers and Futures - hence, to get enough rankings to get into ATP qualifying they need to win more matches and titles than they used to have to.
if anything the part in bold should be better for young players, as comparing to older ones, they are less consistent and then indeed have difficulty to cumulate many tournament good results which their older colleagues are more able to do, that's their main problem at the challengers and futures level,
but when they're in a good period, they can make one great result.
It has always been the main difference between young and old players, and the way many great young players have emerged : by an isolated great result (Janowicz in Paris was another example of that).
I don't know which change of points distribution you speak about, the one from 2009 didn't have the impact you speak about, I don't know about one earlier
And if anything, if you want to help youngsters to emerge, you'd rather have to favour making a few very good results comparing to making many less good results.
All of these things are leading to 18-23 year olds getting stuck grinding down in challenger land, some may stay there, some may find some way to claw their way in and compete at the ATP level, others will give up because the reward financially makes their career unsustainable, so they drop out and go into coaching or some other line of work.
I agree that's the very big and worrying problem at the moment ... which the ATP should prioritarily try to address (and the 2-year ranking system was the worst imaginable system for youngsters !) because it's very worrying for the future.