This is an interesting thread. I own 2 ex-race horses. My one, Fred was raced from 2 yo to 5 yo. He is just now turning 6. He suffered no injuries - luckily. My other, Seku, was put on the track at 18 mos - a horrible thing in my mind. And it backfired. At age three he chipped his right front knee and came up lame. He's now 4 and I am just getting on him, the first time being last weekend. Of course, some of that time off was due to his recovering from surgery and then a nasty fight with some barbed wire (got loose and then tangled in the neighbor's fence). Seku sold as a colt for $75,000 because he was a grandson of Secretariat and looked just like Big Red. He still does. However, I paid a $250 adoption fee for him. He will only be good for flat work, but that's enough for me.
The first thing I did when I got my boys was to leave them the heck alone for about 3-4 months. We did a lot of grooming, a lot of leading and walking, but mainly, I wanted them to learn how to be horses. They didn't know what grass was, nor carrots and apples. Fred refused to be outside at first and I had to lock him out of the barn so he could get some sun. They had worked so hard in their young lives, that they didn't know what down time was. It took 2 months before Fred even gave a try at playing, and now he and Seku are two fools, charging back and forth, biting and kicking at each other, chasing one another. It's a glorious thing.
I'm not putting down the racing industry because I don't fully understand it. But I really believe putting Seku out on the track that early did him in. Of course, this is a lot different than biting a young horse, but it seems that horses should be allowed to enjoy themselves a little before being worked. There's so much time to enjoy them, why take the risk?
(granted, I'm just a pleasure horse owner - no glory of winning, showing or training at all)