skeletal maturation
Just to pick up on a discussion that arose from a topic now deleted... I find this article really interesting. It takes some careful reading, but gives a very persuasive argument for the fact that ALL horses, regardless of breed, mature skeletally at the same rate. The general conclusion is that it is best not to sit on them until they are four years old.
I thought I'd put it up for others, it's a handy one to refer to from time to time. 