If I'm not mistaken, once you're over the age of 18, you can't be claimed as a dependent on anyone's taxes. They see you as an adult and you have to file your own and your parents lose out on that added benefit.

As for the PO Box, I'm not sure about your state, but Federal yes you can list PO Box as your filing address, provided that's where you receive your mail. Otherwise you'd have to list your residence. If you let me know which state, I can get that answer for you.

Regarding income from Onlyfans, all income is taxable, so you'd have to account for that when filing at the end of the year. You may be able to claim some business expenses (equipment for purposes of working, rental of a studio if you aren't doing it out of your home, etc.).

Best of luck in your endeavors.