I stand corrected.
When I was younger I remember that some race horse owners and trainers wouldn’t enter their horses into the Kentucky Derby because they would have to race them the previous year. Two year olds racing before their legs have matured adequately is dangerous. Eight Belles may have paid for this poorly thought-out practice with her life.
After all, she’s born to run, it is her instinct, so it is up to her owner and trainer to care enough about her well-being to prevent what happened. She may very well have had stress fractures before she even stepped out onto the track.
They got quite a few horses in there who had to have their knees worked on arthroscopically... usually to repair the cartilage. Most of these horses had started racing as two-year-olds, and that was supposedly one of the main reasons why so many of them had to undergo arthroscopic surgery on their knees. Beautiful animals, though.
Some of the ones who didn't have the stamina for a racing career ended up being used for breeding anyhow.
Even back in Man O'War's day, they were afraid of racing a horse for too long at too much weight. Man O'War beat the tar out of most of the horses he raced against, and by large margins, but his owners, among other things, were afraid that as he matured, he would be asked to carry a lot of weight while racing (I think it was over 130 pounds). They were afraid of him breaking down, so he was retired to stud while still fairly young.