Of course it's a balancing act though...stressing the bone is important but so is backing off and allowing it to recover before an injury happens. Starting speed work with a young horse does seem to be beneficial to future soundness, but I like to see horses allowed to grow up a little mentally and physically before they actually race, which is obviously more stressful than training. Zenyatta was in training as a 2YO but didn't race until November of her 3YO year, and she always seemed sound and happy and was a lot of fun to watch. But in the years I've been into racing, I haven't really seen any evidence that 2YOs break down more than older horses. I think there are other factors in breakdowns that are more likely than age.