Whether or not Pog is doping, there is a definite trend towards younger riders at the top of the pile. I read somewhere that all the detailed computer monitoring and scientific coaching has allowed young riders to quickly get to a level that took many years of experience for the old-school pros. They also made the point that the greater number of wrecks may also be related to this - you can hone your power, pedal stroke, pacing, diet, etc all you want and you still won't know diddle about maneuvering in a crowded pack. You can only get that with lots and lots of experience.
Anyway it could be the youth factor is the overriding thing now, not the EPO or testosterone or whatever -- recall that peak fitness is at 20. It would not be difficult to get some hard data on this, something like median age of the top-10 finishers of the TDF for the history of the race. My impression is it will be decreasing in more recent times.