EDIT sorry just noticed the date. Didn't mean to bring an old thread up from the dead. Been a little while since I was on here.
Studies have also shown that concussion strengthens the bones of the horse and that a young horse around 2-4 years old shows the best bone strengthening and repairing to shock. So I'm more of the stance of jump them at age four, just basic cross rails and low verticals so they understand what's expected of them and how to do it. Then you the jumping alone for a while.
If you baby them too much they it won't toughen up the bones to help last under jumping and eventing impaction. I'm not saying go out and jump all the time and a lot. Just in moderation.