The same is true for most sites, even Stack Exchange, because of COPPA as mentioned below. If you mention openly anywhere that you're below 13, moderators are obliged to delete your account (since they can't keep your personal information)
> Subscriber certifies to Stack Exchange that Subscriber is an individual (i.e., not a corporate entity) at least 13 years of age. No one under the age of 13 may provide any personal information to or on Stack Exchange (including, for example, a name, address, telephone number or email address).
> Subscriber certifies to Stack Exchange that Subscriber is an individual (i.e., not a corporate entity) at least 13 years of age. No one under the age of 13 may provide any personal information to or on Stack Exchange (including, for example, a name, address, telephone number or email address).
- http://stackexchange.com/legal