Indeed. This just shows that the players aged 20-25 are a step behind the players born in the previous few years, rather than just being about age. It happens. Amazing players aren't born at regular intervals.
What is to be blamed about top 100 getting older: the game that has become super physical? A certain lack of will from this generations' youngsters? Both? Or is it simply a cycle?
Once Federer, Nadal, Novak, etc. retire, the average will decrease, a fortiori - the result of younger players taking control. The Big 4 is just too good for the rest of the field (or is the rest of the field mediocre? That's where the real debate must be), at the moment.
IMO, the overall age is getting older because players are retiring later. Injuries that used to lead to players throwing in the towel can be better managed these days. Players in general take better care of themselves, so they should be able to last longer. The increased prize money means more top players can afford their own physio, and there's a lot more to go around the rest.
Things like Novak's shoulder injury on Friday. He was able to head straight off and get an MRI on it, confirming there wasn't any damage. In the past, he'd have had to guess if could risk further injury by playing with it. Some players would play on when they shouldn't have.
I also think the Big 4 are 10-15 year's worth of top players all born within five years of each other.