I think if you have so many different breeds of horses, race horses, warmbloods, etc, youo will know that the risks of early riding are clear. Research has shown how the horse's bones grow and mature, and Quarter Horses are no different from any other sort of horse in how they mature or when. They need just as much time to mature as all other sorts of horses.
The Quarter Horse reacts just the same as all breeds and types of horses to early riding - they break down sooner and they don't last as long.
In Europe, most people recognize that horses need to wait to be ridden. As a general rule, only race horses are started extremely early there. Things are very, very different in America.
With race horses, of course the goal is to get them racing as soon as possible. If one waits til they are more mature they don't bring any money in just sitting around eating all those years. It's about economics of quick profit. How long the horses last in the race horse game, is not as important as getting quick return on money.
With Quarter Horses in America, the goal is similar - to get them sold as quickly as possible. To do that, they are started very early. There are 2 year old classes at American shows, so they are started three to six months before they hit that age. This is true for most of our breeds, including all the 'american made' breeds - American Saddlebred, Quarter Horse, Morgan, Appaloosa, etc. There are many people who break them very early and show them very early.
Many are started at the same age as race horses - 18 months. For one thing, people are afraid if they wait longer, the horse will be stronger, so they get on them when they are weaker and smaller. But what is done with race horses - where they move mostly in straight lines, is not as bad as all the circling that saddle horses get when started so young.
A few people are able to start young horses without damaging them. They will just sit on them for a few minutes and familiarize them with saddle and bridle, and then leave them alone. In some parts of the world warmblood horses are saddled and bridled and ridden a little at 2 1/2, then left for another six or nine months to mature more in the pasture.
The concerns about physical damage are obvious. Especially working them in a small pen ('round pen') where they are circled in small circles at speed, or 'ground driven' (also lots of circles). Pressure on immature joints and legs from riding too soon can cause arthritic changes to occur in the legs much sooner in the horse's life than expected. This means many horses are 'retired' or have to be put in light or occasional work at six, seven or eight.
There are also concerns for the mental health of horses that are started very early and competed at a young age. Most trainers will tell you that the horses get 'sour' - unwilling to perform and resistant to being ridden.
I've had horses that were worked early(not by me), and without exception, they all developed leg problems by eight and had to be retired early. No, not every horse will be affected that way, but too many will be.
