Every horse I've ever known(stock horses ans thoroughbreds, some arabians) has been started at about two, and I know almost every single one of them is still sound(barring injury) and some are nearing their 30's.
It depends how they are started. If you are getting them used to bearing weight for 10-30 minutes of light w/t, mostly in straight lines, the likelihood of soundness issues from over work is slim to none. The harder you ride them at a young age, the more the chance of future soundness issues.
For the average horse, I start at 3, for a draft, 4