My mother and I were talking about this this morning. Thoroughbreds' birthdays are in Jan. If a foal is born in June, on Jan 1st they are considered one year old even though they are only 6 months. So that's why a lot of breeders will try to breed so that they foal in Jan/Feb. Plus, think about it, a horse is a HUGE animal and their cannon bone isn't that big!!
Years ago my mom got an old race horse that never won, always came in second or third though, when her vet checked her over after looking at her legs he said "this horse has more head than heart!" She became a brood mare, then was turned out to pasture and then my mom got her as an older lady!!
I don't know what board it was on, my mother was reading it this morning, but people would like to find a way to make it a "law" that horses can't be raced until the appropriate age. But with all the money involved I really don't see a chance in he!! for that idea. It would be nice though.
RIP Eight Belles