Thread:GoalScorer49/@comment-25141539-20141005071841/@comment-4196050-20141008113749

It's becase of US COPPA laws. Essentially, American websites can comply with COPPA by either requesting parental verification or blocking all users under the age of 13 from joining. Parental verification can be tricky to authentically verify, so Wikia decided to make their service 13+.