And I'm sure is a 15 year old was carrying some one on their back for athe same length of time as a horse they would have issues,because a human just running around is a good way to help prevent osteoporosis...
20% of a 16 year old's weight is around 35 lbs. That qualifies as a light day pack. In the case of my 16 year old daughter, 20% of her weight is about 24 lbs...which is probably less than her school book bag weighs. A 2 year old racehorse carrying 120 lbs is carrying about 10% of his weight.
At 18, my SIL in Iraq was a machine gunner. IIRC, he was carrying about 120 lbs on a 200 lb frame, except he lost 50 lbs on both deployments so he finished carrying 120 lbs on a 150 lbs frame. Yes, that damaged his shoulders and back - but that is at 80% of his body weight. I grant that we shouldn't allow 800 lb riders on a horse...
There is nothing magical about picking an age where all bones have stopped growing. Lots of 15 year olds have bucked hay for a summer, tossing 100 lb bales all day. I doubt many enjoyed it, but they didn't seem to suffer lasting damage.
Research something called Osgood - Schlatter Disease. Its most common in young people that were involved in extreme athletic activities, especially males...
...but that's totally ignoring the fact that the many horses started young that do suffer as a result aren't managed with such a care - a young immature 14.2 - 15h horse that suddenly goes from being ridden lightly by a 120lb person for a an hour or so a day gets sold on (to make that fast profit) to having to carry a 200lb man plus a heavy western saddle for a whole day likely isn't going to do so well
[Note: Osgood Schlatters disease typically consist of pain at the tibial tuberosity or bony bit at the top of the shin....The disease is a very common cause of knee pain in children and young athletes usually between the ages of 10 and 15. It occurs due to a period of rapid growth, combined with a high level of sporting activity. Osgood-Schlatters Disease - Symptoms, Causes and Treatment
First, a two year old horse is physically more mature than a 12 year old human. It is not a period of rapid growth, and in fact the bones HAVE fused in the lower leg.
Second, if you do ANYTHING wrong, you get bad results. As I pointed out, if you ride with contact poorly, you create all sorts of problems. Does that mean we should ban English riding? And an English saddle distributes weight over a smaller area - does that make it cruel?
Mia is tall for her weight and leggy. If I tried to push her to be competitive at reining or barrel racing, it would be obscene. But I'd bet her
back could handle endurance racing if mine
Western curb bits work well with Mia. If I jerked the reins, rode with contact and never gave her relief, they would be brutal. Used properly, they were the single best decision I made about tack on her.
In like manner, you can ride a 2 year old in a way that trains them or in a way that destroys them. How much that differs from riding an 8 year old horse, I don't know. I am pretty confident that growth plates in the back are not the critical factor!