Rafa's truly unbeatable years on clay was exactly when he was super young. 2007-2008 as well as a portion of 2010 (although he had already lost some movement after the injury). This isn't that surprising, most great players peak on natural surfaces at much younger ages. Rogers truly unbeatable years on grass was 2003-2005 and even though he was still dominant in 2006 onwards, it was never quite the same. Sampras was the same way, Borg was the same way, McEnroe the same. etc etc
You learn more as you get older, but youth has stamina/movement advantages as well as somewhat paradoxical advantages in the mental department relative to how you are as you age.