I mentioned this before and was ignored:
https://en.wikipedia.org/wiki/Roger_Federer
That's 12 slams between 2003 and 2007. Age 22-26
4 slams between 2008 and so far in 2015. Age 27-33. Still impressive, but nothing like his peak.
There are other stats that show that Fed was at his absolute peak before 2007. So in 2008 he ran into the wall that all great players run into.
https://en.wikipedia.org/wiki/Rafael_Nadal#Grand_Slam_tournament_performance_timeline
Arguably the greatest clay court player of all time has three FOs by 2008, nothing else. 6 more from 2008-2014.
But all non-clay slams were won between 2008-2013, age 22 to 27.
Two great players with two different peaks, both from 22-27 or 28.
The five year difference in age explains most of it. The fact that the two excelled on opposite surfaces explains alot of the rest.