The answer may not be that you do not need anything because you are a minor the answer probably is that you can not because you are a minor. So to answer your question directly you can not legally operate a business based on the information you got, not that you can somehow operate and not need to do anything.
The town answer is average, most towns do not require anything. The county though is to protect you. If you go to the link I gave you it specifically addresses all the issues you asked about including the county DBA reg.