You can start riding horses as young as long yearlings. I would recommending to wait until they are two. Its not how young you ride them, its how you ride them (have had countless vets tell me the same thing). If you start your horse at two ride them for no longer than 30 minutes. 15 minutes is best, but sometimes as we all know, we can't get what we need done in a set time. Don't just go in a circle have them walk and trot all over the arena, as you watch a horse outside they don't keep going in circles or straight lines very long. As you also watch youngsters outside they are pretty rough on themselves all the running and bucking, kicking, rearing, etc. So, if you start them at 2 just remember its not an endurance ride, get on and get off within 30 minutes (preferrable 15).