One more comment since I saw a few others mention athletes who were great in their mid-40s. Randy Cotoure, who at the age of 44 came out of retirement as the smallest heavyweight in the UFC and took the belt from Tim Sylvia. If you ever want to see a performance give you goosebumps watch the opening round of that fight.
He defended the belt six months later against an even more dangerous opponent who had 30 pounds of size on him in Gonzaga. #oldguysrule