Conveniently ignored in all the stories about teenage whiz-kids who make money off internet businesses is that they are invariably committing federal felonies. There is not a single payment processor that doesn't require its users to be 18. Apple's developer program also requires its users to be 18. So does AdSense. These services also require users to input a birthday, so the minors using them needn't worry about "hacking" charges - they've committed fraud.
Obviously we need to fix the legal situation that makes this necessary, but that's why. (And because my parents follow rules, I wasn't allowed to do anything vaguely entrepreneurial on the internet until I turned 18).