Most of the time, in showjumping, it's 3 years old. We prepare them and they are supposed to start small competitions at 4, during a small season, without the final as a goal. It gets them used to travel, to see other arenas. I think that young horses competitions are very well. Breaking them and working them late isn't a good idea, the younger they are manipulated (not necessarily ridden), the most clever they become. We see it in sport in general (tennis, football,...), it's easier when you start early.