Something a lot of people don't know is that it isn't just the knee joint you need to worry about. The joints involving weight baring in the front legs are not finished maturing until 3-3.5, 3.5-4 for the shoulder, and the hock is not fused until 4 (which is why you shouldn't even start thinking about jumping until after 4). The last thing to finish maturing is the horses spine, which doesn't finish until 5.5, and is later in larger horses with longer necks. It also takes roughly 6 months longer for male horses' to mature.
Keeping this in mind, I personally won't get on a horse under 3 at the earliest. When I start a young horse, their 3 year old season consists of light, short rides and lunging sessions in a soft footed arena, or short slow trail rides. I don't find any sense in risking a young horse's health and soundness just so I can ride it sooner.
When after all, there is a LOT of things you can do with a 2 year old to prepare it for being ridden, and doing whatever disciplined you intent it for. Tying (even ground typing), standing to be groomed, feet cleaned and trimmed, clipping (even if you don't actually cut any hair you can get them used to clippers), bathing, trailering, in hand manners and showmanship (backing, trotting, stopping, walking turning, etc.), if you have a quiet broke horse you can pony him on trail rides, saddling, bridling, lunging, ground driving, teaching them to yield to the bit and the correct way to respond to pressure on their sides, in hand trail, in hand shows... the list goes on and on. If you do all this with a 2 year old, by the time the horse is 3 it is basically broke and just has to translate to actual rider cues. Which, if the ground work has been done properly, is almost instant.
Does this mean riding a two year old will make it predisposed to lameness in the future? Not necessarily. But that's not a situation I knowingly would put my horse in. I hope this doesn't come across as condescending, I was surprised when I first learned this. Just something to think about.