Contrary to popular belief, different breeds do not mature at different rates. Some may LOOK more mature, more muscular, whatever, but SKELETALLY they all mature at the same rate, give or take maybe 6 months. Stallions mature a bit later than mares. All the bones do not finish growing at the same time either. There is a progression that happens until, finally, at around age 6 the horse is skeletally mature. People use the knees as a guide, while ignoring the rest of the skeleton. I don't understand why that is. The knees, oriented vertically are much better suited to bear weight, than the back. So why not wait longer for the back to mature?
That's not to say you can't teach them a whole lot of stuff in the meantime, including saddling. I use a saddle pad and a wide elastic blanket surcingle as a first "saddle". I hope that by the time mine are old enough to ride they've been there done that and all that's left to do is get on.