Im with zac myself.
Basically all physical attributes are on a decline from that age, and if your constantly at your physical peak, your going to go downhill.
everyone makes a good point, but in effect most people have the advantage at a younger age. If you disagree, just look at any competitive sport. sprinters, swimmers, billy goat wrestlers, whatever the sport, it is mostly dominated by younger folks.
Boxing, look at Ali or Frazier, Roy, anyone. they all got old
As mattj said, of course there are exceptions, where mentally someones skill evolves and actually improves them, but 99% of the time it doesnt happen.