Originally Posted by Netspirit
30 is the age when the spine loses flexibility and the blood vessels in it start to get "fossilized" with calcium (so the spine essentially starts to die and this process is irreversible until the very death of the human).
The muscles lose their ability to contract explosively, the brain loses its reaction time, the eyeball muscles begin to degenerate which decreases the ability to track fast-moving objects, the vision itself gets worse, and the stamina is depleted much sooner than during the peak 16-26 age.
The fact that Federer can still hang on with the young and best players in the world is remarkable. The fact that he even beats them from time to time is a biological anomaly.
Geez it's not like he is 60. Fed's in great shape for 30, but there are lots of other pros who are still playing well at that age. Look at David Ferrer, playing the best tennis of his life at 30. Tommy Haas is 34, and still top 20!