You can start training a colt under saddle at age two, but not putting them under serious work or turning at high speeds. I have started two year olds many times and never had one end up with problems. We purchased a colt that was a runt and late colt (born in July), and by his two year old birthday he had grown quite a bit, but not near the size and health I wanted him to be. We waited until he was a full three years old before ever saddling him....he's riding out fine, now, and is a perfect angel. He's gained a lot more muscle, though his height will probably remain around 14 hands. I think it really depends on the horse's bone structure, attitude, and overall health. You can start a colt at two, but no hard training. Just work on the basics and nothing that will stress their knees. Doesn't hurt to put a saddle on them for a half hour a day to get used to it, too. But if they're good sized, you can start riding them, and just take it easy. No cutting, reining patterns, jumping, or other stressful work.