The oldest horse we have at the ranch is a 33 year old ottb. He's still sound. He was retired this year, but still comes out for the occasional lesson. Actually, he let out a playful buck a few weeks ago and left his student on the ground laughing. He's not on bute or anything. I recently started my friends filly for her. She's a coming 3 year old. I rode her twice a week for her first 5 rides. She had about 2 months off and I just recently put two more rides on her. Now, at ride 7 she will stop with my seat and voice, yield hind quarters and forehand as well as side pass. Actually, I tried closing a gate on her last ride and by golly she did it!! Imagine that. She has trotted a little bit, by little bit I mean twice around the round pen is the most in one ride. Usually only a few steps. I don't think I've ridden her more then 20 mins at a time, at the very most! Training is about quality and effectiveness, not the amount of time when it comes to babies. I have a coming 3 year old at home who I'll be starting this summer. I plan to start her, riding about twice a week for very short rides. And then giving her the winter off. There's nothing wrong with putting a good, light foundation on them and then letting them grow up IMO. As nrhareiner said, I'd rather go slow and easy then have to push hard and fast. That's how you end up laming a horse, no matter the age.