The subject of underage users has popped up a lot in the chatroom lately. Wikia's official stance on the matter is that wiki admins are not tasked with enforcing this aspect of the Wikia Terms of Service but admins may enforce or report violations of the Terms of Service if they wish.

Wikia COPPA statement

"We don't ask our Admins or users to patrol this aspect of our Terms of Use, but we are more than happy to look into reports sent in via Special:Contact."
— 2013 August 15 by Semanticdriftr