I have 3 percherons myself. It is okay to start them younger than 2 if you don't push them too hard. When you ride, only go short distances at a moderate pace (no racing or running hard). Most horses are able to be trained young so long as there is an experienced horseman doing the training. A good trainer will know when it is time to stop and when the horse has had enough. More often it is not the horse's back that you need to worry about, it is the growth plates in the horses legs. They can become deformed in a young horse that spends too much time in a roundpen at a hard trot. If you are going to start them that young, ride them for a few days in the pen and then take them on the trail. Also, at a year and a half, most draft horses are about the size of a small quarter horse. If the animal weighs 1000 lbs, its human companion is not much more than an afterthought.