ALL horses, regardless of breed or outward appearance, mature at roughly the same rate - reaching full maturity at 6-7 years old. Check out Dr Deb Bennett's excellent article which completely explains the process, with the scientific research to back it up.
I, personally, would be very hesitant to buy a young horse who has already been jumping so much, rider or otherwise. I agree with whoever suggested xrays as a part of the PPE.
Height is not really the issue with jumping, frequency is. As young horses, we generally do not get a whole ton of indicators on athletic ability and what their final potential may be. Generally speaking we don't truly know if a horse is a GP horse until they get there. But things like jumping scope and movement can be evaluated at a younger age, while confirmation will change until they are done growing. So we do things to challenge the horse's ability while it is young to vet an idea of if the horse might have the things we want to do the big stuff. Young horse inspectors have the best feel of this, so inspection results are a good indicator of how a horse may do later I life. And that does involve jumping some big stuff as 2 and 3 year olds.
That being said, they shouldn't be frequently worked hard at 3. Imo 4-5 tines a week for 20-30 mins is lots. But one can still do a lot in that time, and shouldn't stagnate the training.
In dressage, expecting the horse to be at a moderate level with flying changes and collected gaits by 6 is bot unreasonable with a talented and athletic horse. To be at the small tour at 7 or 8 and the GP at 10 is I would say normal progression for a talented and athletic horse. I find far more issues with stagnant training, because when you go to move on, the horse objects and throws a tantrum because he was never asked to put in effort before. Behavior problems start when we think "oh nice horsey didn't buck me off - good boy!". Ask for improvement and change every ride. Keep the brain engaged.
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That might be the trick here. IF he is only being jumped infrequently it may not be doing the damage. I am at a point, however, where I am not sure I want to take on a three year old even though the owner swears this horse is a as good as gold for anyone and everything. 20 years younger in my age I might be ready but right now, I think I want a horse a little further along. I have an excellent lead on a 12 yr old Belgian Warmblood mare already showing second level with scores in the mid 70s (confirmed at centerline scores) and schooling third level. Still, this three year old is very cute and has great potential as a dressage horse.
I made the decision to pass on this one. While the horse does ave the potential to go high in the levels with the correct training at the right time, my instinct here is telling me no. For one, I got the conformation pictures and while, for once, he is a horse with good hind legs, his front legs look...strange. Pics are difficult to a certain degree depending on how the shot was taken but the right front looks like it angles outwards..as in not set square at the knee. Given the early jumping and the three years he has yet to mature, I am passing.