wouldn't be a bad idea but how would it be regulated? some sites i'm on usually have a combination of the year you say you were born when you sign up, and purchasing an adult forum pass for 3-5$ depending. granted, this is the internet, some kids might think that the adult forum would be full of us talking about magic to turn them into vampires which we refuse to share or something. so they make a new account claiming they're 21, borrow their parents credit card, buy access, and boom, same problem but hopefully on a smaller scale.
using mail or joining a more exclusive coven is your best options right now. or just ignore people who ask repetitive/fluffy questions.