I was able to crush my competition at a very young age by starting out working for free. I just made sure my contract with the business was air tight. After the first few projects where done I was charging $120+/hour before I was 18, and well beyond that after reaching 18 but charging either weekly or by the project with a very tight contract to insure both parties knew the scope of work and the costs and requirements for work requested outside the scope of the contract.
It is hard for someone to pay a 15 year old any amount of money for a site, logo, service or app without them having a portfolio of corporate clients to use for reference, their own sanity and justification to their other business partners. I would say the only way to do it successfully, is to have a contract in place so you can keep the work reasonable for a non paying customer.