^Cat, horse's backs & hocks are still 'open' until around 5 yo at least. IMO riding a 2yo or riding a 3yo is little difference. But I do think it depends on how much riding, what sort, how heavy the rider, etc, as to how potentially damaging it can be. I would advise very little weightbearing at all on a 2-3 yo & only 'easy' riding, no big jumps or high impact on a 3-5yo.
Understanding the reason behind the worry, & the effects, can help us decide how much/young/what sort of riding is appropriate.
Here in Sweden we had a standing rule of thumb, "one should not sit up untill the horse is into the 3´d year".
Military Remounts where not accepted untill four years of age where they were would remain under care and ground training. Not untill the 5th year was the remount issued to the regement for service.
We have the same rule in Endurance Competion no horse may start that is not of 5 years of age, figuring from January.
Personaly I would rather give todays fast growing horses an extra year before starting.
I believe you are referring too the Growth Plates loosie, the shock absorbing like cartilage laying between the bones. These need time to develope and in the final stage harden, what we call Drying Out.
Some very good reading if I can get it too work (may need your help!)
google, Timing and Rate of Skeletal Maturation in horses. By Dr. Deb Bennet. http://www.equinestudies.org/ranger_..._2008_pdf1.pdf
And YES, shoeing should be held off for as long as possible or untill the horse is fairly developed.
My neighbors quarter horse had been shod at 2½ was a size 00, when he was 5 he had a size 1?????? and it is said the foot is the first too finish developing?