Horses do not reach skeletal maturity until age six. In races in Europe, horses are not ridden until age four, but here in America we seem to be in such a hurry. No wonder we lose so many racehorses at such young ages!
The pastern growth plates will be fused early enough for early riding, so whatever it is now may be as good as it gets. Snap a still picture, and look hard before buying!) The biggest problem is the back, because the vertebrae don't fuse until around five and a half.
This is from a race horse trainer, but it can be gleaned for important information on horse maturity:
http://www.webertrainingstables.com/ridingyoung.cfm
The Significance of Too Much Too Soon.
What will happen if you put a young horse to riding much too early Two important things - and probably not what you're thinking of. What is very unlikely to happen is that you'll damage the growth plates in his legs. At the worst, there may be some crushing of the cartilages, but the number of cases of deformed limbs due to early use is tiny. Legs can be damaged much quicker and more seriously by over feeding a young horse.
Structural damage to the horse's back from early riding is somewhat easier to produce than structural damage to his legs. There are some bloodlines that are known to inherit weak deep inter vertebral ligament sheathing; these animals are especially prone to the early, sudden onset of "saddle back'" However, individuals belonging to these bloodlines are by no means the only ones who may have their back "slip" and that's because, as mentioned above, the stress of weight bearing on the back passes parallel to its growth plates as well as parallel to the inter vertebral joints. However, the frequency of slipped backs in horses under 6 years old is also very low.
When trying to explain the justification and common sense of waiting until at least 3 years of age to introduce a horse to light work under saddle and incrementally increasing that workload over the next year to a competitive/ performance level by age four, I am almost always inevitably confronted with an impatient disapproving owners condescending response in notifying me that race horses have been started early for centuries and are already often competing at age two."
And another, which explains the anatomy in detail:
http://www.equinestudies.org/ranger_2008/ranger_piece_2008_pdf1.pdf
I have a friend who is an endurance rider, and she likes to start their heavy training at eight years of age. She wants her horses to be lifelong companions, and because she is patient, she runs into very few problems with soundness. Of course, endurance riding is a LOT more stress on the horse than regular pleasure riding, so four is plenty mature. Two is pushing it. I've heard of so many good horses blown out early, even big, sturdy Quarter Horses can be ruined started too early. Entirely your decision, but please read about the anatomy before starting.