I agree with the idea of waiting for horses to mature to begin their training, but this is true for show horses as well as racers. I have seen so many halter horses over the last twenty years that have been pumped with feed and suplements, and I suspect steroids that it is just sickening.
A yearling looks like a three year old and the three year olds look like fully mature horses and by the time they are five or six so many are limping around with bad feet and so heavy they can hardly walk. And the riding events are not much better, there are a lot of two year old snafle bit futurities with horses being forced to do things their bodies are not ready to do. Some one has already said that horses love to do what they do and I can speak with forty years of experience that this is true. Put twenty yearlings in a large trap and watch them race each other, they love it. And I have know bull dogin' horses that would nearly cry if you loaded up for a rodeo and they could not get in the trailer.
The problem is as, again, already stated, starting them too early, and breeding for small bones and long strides. Racing Quarter horses don't break down at the same rate as Thouroughbreds because they don't run as far and have been bred with larger bones and shorter strides. But they still start them at two and they do break down too often.