User blog comment:Wikian13000/Its illegal to be on here under 13?!?!?!?/@comment-944111-20101027015750/@comment-60868-20101027023033

Reason is due to what content you find on the sites... 13 is a reasonable "young adult" age (see PG13 ratings). It also means that you can upload pictures and consent to their use on the sites. 13 is the normal age to create logins and state that you agree to terms and conditions (and be bound to them).