Here in the UK they can start competing around the age of 5, but mostly they are about 7. Lots of kids here start competing before they can somersault, and the club levels and the first regional level doesn't have to include somersaults. I think if they start competing earlier they are less bothered by the whole process when it gets a bit more serious, they already know how it works because they've done a few comps that didn't really matter. If they start at a higher level they have to worry about a tough routine and the fact that they've never competed before so they don't know where to go or what to do.