Hey I'm as big a Steemit fan as anyone else but personally speaking building a business or even more importantly your life on a third party platform you don't control to me isn't a good idea.
While I love this site and am very optimistic about it, I think it's too early to be planning the rest of your life around this platform.
I think a 15 year old would probably be better served getting a job, learning to work with others, learning to take instruction from a manager or boss and being around their friends.
I'm not saying it has to be one or the other, a kid could very easily have a part time job with the park district or mowing lawns and also do Steemit, I don't think it needs to be one or the other