In the UK, races have upper weight limits and usually with height, goes weight. We have had several young jockeys who as they get older, if they keep growing it becomes harder and harder to stay within the weights required and stay healthy! Aiden O'Brien's successful son Joseph, had to give up race riding, even National Hunt (although I don't think he was keen on going over jumps?) because he just grew and grew. He is now training, successfully too.
Horse racing jockeys are not disc jockeys (unless in their spare time!?)
This doesn't only apply to the better known Ky Derby!