Varies horse to horse. We tend to start jumping our youngsters when they are 5 (summer of) as we like to make sure that they are mature enough to do so.
Some growth plates in their bones don't close until 6 years old. Jumping them high young can damage their bones so take it slow, keep it low and not too often
i used to show in small greens, and my pony was 4. i don’t really regret it, but i think it would be fine if i started a little later.