Depends on the player really. Just been reading that Ledley King is contemplating retirement at the age of 31, but he's been plagued by injuries all his career so perhaps that's no surprise. Not sure how old Michael Owen is, but he's obviously still capable of finding the net at the top level. And wasn't Gordon Strachan, always known for his commitment to fitness, still playing at 38?
In general though, professional athletes have shorter careers these days, You see it in cricket and rugby as well. Ironically, increased levels of fitness lead to greater pressure on the body and a greater risk of injury.