bussiness

If it's a "babysitting business," I don't see anything wrong with an 11/12 year old doing it (although, if they wanted to make it official and get a business license, I imagine an adult will need to get it for them).
If it's a 10 year old that wants to own a clothing business or a 9 year old wanting to own a restaurant...well, I think that's just wishful thinking. That's a dream they'll either outgrow or hopefully achieve when they're an adult.