That's not at all what I said Rob and I think if you re-read you'd realize that.
To clarify I will say that you CAN have a child/horse/animal that is mature for his/her age. However, you CANNOT push any animal to full maturity at a faster rate than they are biologically programmed to do. No matter what conditioning program you use.
Do you let any animal rot in a stall or on a couch until they are fully mature? No, of course not, that leads to as many problems as asking an immature body/mind to do too much before it is ready.
What you should do is help the body and mind be as mentally/physically fit as is possible for their age. You wouldn't ask a 7 year old child to do the task of a 15 yr, but you would begin laying the groundwork in order to give them the tools necessary for the task. Same thing with young horses, you should be laying the groundwork to enable the horse to complete the tasks that will be asked of it when it is mature. But the basic timeline for maturity isn't negotiable.