I got my horse when she was 3 and she was already jumping 3'
And this is why we have horses who end up lame before they should be.
I highly disagree with how riders in the Adult Ammies have 2,3 year olds jumping 4'0" already.
And that is why, we see many horses turned out to pasture with lameness issues, before they even hit their prime.
Now on that note, I do not disagree with riding babies slow, steady and educatedly. There is nothing wrong with putting them over trot poles, cavaletti's and very small x rails once in a while - while focusing primarily on ground work and flat work.
The horses welfare should be put into consideration 1st and foremost. Regardless if their joints are fused or not. It isn't about how much they enjoy it, it isn't about how well they are doing at that age - it is about the long term effects that will show, and I promise you they will - when they get older.