Great question!
As a mom, you know your child best. There is no magic number. However, I think that waiting until they can practically drive themselves is a bit stifling. So, don't wait until they're 16 or 17. 
I've seen kids getting into some trouble when they're too young. We worry about everything from them getting lost to them being poorly influenced to being kidnapped (or worse). As long as you know your child's temperament, wisdom, interests, and street smarts will serve them well, I say you let them go. And make sure they exercise safety (know the area, know the rent-a-cop location, go in a group, take a cell phone, etc).
Keep them home if they're rebellious, foolish, disinterested, slick, and generally unobservant. Also consider if you owned the shop, would you see that child as a customer or a liability?
