I hope I can help you with this.
That’s correct. You need to be a legal adult according to the laws of your country, so the required age depends on where you live.
Since this is a program to monetize your island and it is strictly specified that you must be of legal age, I wouldn’t risk using your parents’ account. This could violate Epic’s rules, and as a result, your account could be suspended.
I don’t know how long until you reach the legal age, but in the meantime, take advantage of this time to absorb all the knowledge about the engine and apply it to your awesome island 
Here is the information Home | Support-A-Creator
Have a nice day!