It depends on the sport. An athlete doesn't reach peak performance at 28. I don't think we even reach muscle maturity till our 30s.
again, highly subjective.
I think an mlb player's prime could go well in to their 30's than say an nba player whos prime would be mid 20's, because at that age you recover from inuires quicker,more stamina, alertness and overall just being younger=healthier