There are addons that cater to it. I had a nifty feature added to @Xon 's Moderator Essentials which ban users after they fill in their date of birth. In my experience most underage users do not see it coming in this context. And are later banned because of it. If you combine it with a good multiple accounts protocol then you have a winner.
Users are automatically unbanned when they turn 18 years old. You can set a grace period, so that you do not ban people who are about to turn 18. https://xenforo.com/community/resources/moderator-essentials.1269/