User blog comment:M1870/13 and Up???/@comment-1032852-20120522043714

The Terms of Use state that "the Service is intended solely for users who are thirteen (13) years of age or older", and anyone under 13 that creates an account is violating said terms. This exists so Wikia can conform to the Children's Online Privacy Protection Act (COPPA). Underaged users may be blocked so that Wikia is in accordance with the law, no matter how many contributions the user has made or how they act while using the service.