not sure if anyone else in the thread has mentioned it, but i think the physicality of players present near the top of the game says a lot.
it's become harder to bridge the gap between talented young guys and talented top players because the added dimension of physicality, not just muscle mass, but degree of conditioning. it takes time to build top level conditioning and money to get a trainer to do so. it seems within a couple of generations, its become a lot harder for a pure shotmaker with a typical 18 year old tennis player's physique to shoot up the rankings.
if the climate of tennis (technology, court speed, style of play) stay the way they are or keep moving in the same direction, the breakout age of 17/18 that we're used to seeing regularly before, will become more like 23-25.