This isn’t commonly known but horses legs are not fully developed until age three. The rest of their bodies don’t catch up until age seven. Nevertheless, the stress of Thoroughbred racing at age two, which is the age required at the Kentucky Derby, can result in the terrible tragedy that occurred Saturday.
As an example of athleticism in horses, Lippizaner’s perform incredible athletic feats - but they don’t start training until age seven and their training takes as long as seven to eight years. Many of these horses, all stallions, perform well into their twenties.
Actually, the Kentucky Derby, as well as the other races in the Triple Crown, is for three-year-old horses.
There is also a genetic component to these failures. The Friday Wall Street Journal had a story about the the inbreeding using the genetic stock of Native Dancer and how it has affected recent horses like Barbaro.