Only if there are good enough players to challenge them. For all intents and purposes the only player who can challenge him is practically of the same age. So there's no age advantage there. Everyone else sucks on clay...or is made to look sucky compared to the supreme talent of Rafa.
I guess I just tend to be more cautious when it comes to predicting the future. Rafa won his first RG title when he was 18 I believe. Are you telling me there are no 16-17 year old amateurs right now that aren't a year away from going pro and beating a 28/29 year old Rafa at RG? Michael Chang won RG at 17.
Who could have predicted at the end of 2010 that Nadal would lose 3 straight slam finals against Novak in 2012? Whose to say Murray doesn't get his head on straight in the next year and beat Rafa at next years RG?
Nadal is 26. Your physical peak is 25. He takes great care of himself and is exceedingly disciplined. But his game is extremely physical. It is not realistic to think that at age 28 or 29, he will be as dominate as he is now. It is also not realistic to think that no one is going to appear on the scene who can challenge him over the next three years. That's a long time in tennis.