Here's Terrell Owens, who turns 39 in December:
Wow, look at all that lost muscle mass!!!111
Um, football, baseball, basketball, hockey.
Ever heard of them?
Anecdotal evidence means nothing. A man at 70 can take steroids and build muscle.
Like it or not, the body produces huge amounts of Growth Hormone and Testosterone up to a point where it slows down. This varies with everyone, 30 is not a cut off date by any means.
You see some teenagers with tons of natural muscle and body hair, then ten years later they are burned out, they got a huge amount of Testosterone release early. Others get less but over a longer period of time, they last longer. Same deal with Growth Hormone. We grow until we start to devolve, at what age for each person is different.
I read a fascinating study by scientists who say that 50 IS the new 30. The reasons they gave were too many to name, but you can see it. Look at any "Twilight Zone" from the early 60's. Serling always gave the persons ages, "This is Joe Smith, 29 years old", we just laugh, because the guy looks 50. Do you look as old as your dad at your age? Look at Tom Cruise, John Stamos, 50 and 49 respectively, but you wouldn't guess them over 30, and it's more than anecdotal evidence, we see this everywhere, of course with exceptions.
30-35 most athletes involved in reflex sports highly dependant on eye/hand coodination like Tennis and Boxing quickly devolve, for the exact reason the original poster said, the nervous system gets "Burned out".
A giant defensive lineman, barring injuries can last longer and be more efficient then for example a Tennis player.
High performance boxers like Joe Frazier and Wilfred Benitez were shot at 26, same deal with some Tennis players. Others who didn't have styles that depend on contant high energy and performance last much longer. Hopefully you can see how that translates to Tennis. The Nadals burn out quicker than a Federer because of playing style.