Honestly I like the way my sister is doing it with her young horse. He was broken as a 2 1/2 year old (my opinion on this, it depends on the horse. He was "horse shaped" and not "baby shaped" and all sessions were short and basically twice a week) and as a 3 year old did a lot of basic dressage with trot poles. Later into his 3 year old year they free jumped him over some grids (took him up to 3'6" and that was as high as it went) so he could figure it out and develop some muscles. Also he started doing raised trot poles and developing more muscle, which was a touch and go thing as he would just lose a lot of it as he grew. Even later he did small cross rails, which he didn't actually jump, just used as raised canter poles to learn striding and such (haha to give you an idea when we were free jumping him once he used a 2'9" jump as a raised trot pole). Now as he's 4 he's going to be doing <3' until he is 5, and then he will start jumping higher. That's the way I'm going to do it when I have a young horse, of course varying slightly depending on the horse.