When it comes down to it, it's not a matter of if a horse will break down, but a matter of when. The age of the horse when you start them may only extend the time it will take for that horse to start developing lameness and degenerative bone disorders, it does not prevent it. This is due to the inherent intensive nature of most riding, but in particular performance riding, which is inherently destructive to their musculoskeletal system. As a quick example, just look at the over-extension of their joints when show jumping and imagine the effect of doing that regularly:
It’s the same even with groundwork-based riding such as reining, dressage, racing, or any other moderate to high intensity horse sports. The horses legs and joints are repeatedly subject to suspensory and torsional forces with the addition of extra weight on their back. It’s no wonder lameness is one of the most common issues (inapparent and apparent) in ridden horses worldwide. Riding is a major catalyst and cause for the disease.