Here we go, this is what I was looking for:
| Bone Development |
The primary consideration when deciding the best age to break a horse is bone development. A foal is really no different from a newborn infant; young horses' bones have not yet fused entirely into an adult skeleton, and are therefore subject to breaks and fractures.
Starting horses under saddle before their bones have fused can cause numerous injuries, including permanent lameness. It is best to have the bones X-rayed before breaking a horse to make sure those bones are ready for the weight of a rider.
Breed of Horse
The breed of the horse has an enormous impact on when one should start a horse under saddle. Some breeds, such as quarter horses and appaloosas, can be started at two or three without any problems. Others, such as Thoroughbreds and Arabians, should not be started until they are at least three (preferably four). The reason, again, is in the bones; horses with finer bone structure should not be ridden as young.
Some horses take longer than others to "fill out". Much like human children, horses will begin to develop more muscle mass as they age and will hold their grain easier. A lighter horse is more difficult and dangerous to break because he is more fragile.
Source: Best Age to Break a Horse: When to Start Horses Under Saddle
I wanted to post my little opinion first, but when I saw that first article I couldnt focus. Lol
I agree with the stuff I quoted in this post.. I don't think I could phrase it better if I tried :p