I would suggest going to get a job. Nothing against Steemit but I think at your age it's important to build some work ethic and get used to taking some shit from other people. Get a job landscaping or working at the park district with othere kids your age and do Steemit in your freetime.
I don't think you would legit be putting in 8 hours a day on Steemit, I doubt anyone could put in a full 8 hours so better off getting a job and posting in your freetime. It may even give you some experiences and content ideas to write about ie a post about my first job
https://steemit.com/@theprophet0/transfers
He is 15 years old too and took a Mentor, and is a Witness. I am not sure if Mark Zuckerberg or Larry Page ever had to experience shit from other people, you can also develop good work ethic on steemit, there are great people at steemit, we are not a holiday camp :D
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