Thanks for your feedback. I think this was an excellent explanation.It does, but the difference is that football is a team sport, whereas tennis is an individual sport that is totally based on movement and eye hand coordination. That is why the retirement age is much younger in tennis; it's because a physical decline plays a much bigger role in tennis than it does in football.
It isn't TOO evident in boxing, but you can clearly tell once a guy hits his mid 30s in boxing he changes his game to a more power base striking game rather than finesse/footwork. He has to; there's no choice. That's why guys like Foreman, Tyson, and other punchers age better than guys that rely on finesse, technique, and strategy to win.
Tennis is never about pure raw strength like other sports; it's about skill, finesse, speed, agility, and eye hand coordination. The fastest thing that degrades in most athletes is your eye hand coordination and explosive first step, both which are vitally important to being a good tennis player. Federer and Roddick are probably just as fast as they were in their younger 20s. The issue is that they aren't as fast off the first step anymore, and that's the key difference.