Originally Posted by themacpack
It really depends on the individual horse, OP. Some horses are capable of handling that sort of thing at 2-2 1/2, others are not. If you think about it, some 3 year olds often appear more like a gangly 1 1/2 year old, and some 2 year olds look and behave like 4 year olds. This is where having a good, solid base of being able to judge that will come into play. Do you have a particular horse you are preparing to start? If so, providing photos would help to at least give people an idea of the horse's physical readiness (glaring "not ready" signs will be pretty obvious - even though one can't see the internal structures).
My horse is one and a half and I wanted to try riding him in late summer. His birthday is in may and he is really big already.