Agassi was 34 years old and took Federer to 5 sets?Your point?
Agassi hardly declined he played very smart tennis. Roger is not anywhere near as good as he used to be. the point is that a mediocre Federer (compared to his standards as far as how he used to play) still beats everyone and can handle these guys in their primes. it obviously says something about his longevity
you won't see Nadal, Novak, Andy playing as well as Federer does at 31, if they're still playing. granted none of them were nearly as good in their primes (right now) as Federer was in his. except maybe Nadal on clay and Djokovic in 2011