What competition? There is no legal age prohibiting young people from competing in competitions, so the age limit is probably only limited by the contest rules.
normally no you can do anything you want whenever you want some competitions (namely bigger ones ie x-games, big air and style, etc...) there is an age limit, not sure, but i think it is 15 maybe...
The intended audience of jousting competitions was members of nobility. But most of the time nearly anyone could watch. The competitors who jousted were probably all members of the nobility.
when your horse is ready, if the horse is green broke at 4, then give them a year, and you should never start backing a horse (training to saddle) before the age of 3 b/c they haven't developed enough