User blog comment:M1870/13 and Up???/@comment-3391671-20120522052350/@comment-4674838-20120522055519

The usual way underage users are identified is when they share their age on the wiki -- usually by posting it in chat or on their profile. On some wikis, local admins will ban underage users on sight for violating the Terms of Use (especially if they have a local policy about it).

Nobody really goes around actively searching for underage users though. It's not a local admin's job to police the Terms of Use. And for that matter, Wikia staff don't exactly have the time to be doing that either.