I don't understand the "it depends on the horse" view.

Science has confirmed that the bones of ALL breeds of horses grow and finish at the same rate. You don't need to have trained ANY horses to read and understand the scientific literature of the maturation of horse bones.

Some horses, the kind and gentle souls that they are, will SUBMIT to just about anything. They are conditioned by nature to HIDE weakness or GET EATEN. We are predators to them unless we spend a heck of a lot of time overcoming that notion to the point that they trust us. Hopping on them at 2 years old doesn't (in my mind) feed into that trusting relationship.

It takes time for bones, ligaments, joints to break down from wear and tear. But that's what they all do, break down. Even in our own bodies over time. So why risk accelerating that breakdown? I could understand way back when horses had a limited life span from other diseases and predators. And they had a specific purpose and had to "earn their pay" PDQ. But in the life of luxury we currently give them, with all the benefits of modern science and compassionate owners who want to and can afford to keep them beyond their "usefulness", they are living longer lives. This longer liife span has not caused the bones to finish maturing any earlier.