ZetaBoards Forum Terms of Service said:
12.1 COPPA Requirements:
You are required to protect the privacy of users under 13 years old. There are three types of registration for users under 13. You may select the type that fits you community and needs best, but you must enforce rules related to the registration type you select. You may not suggest or encourage users to lie about their age to bypass any COPPA registration/profile restrictions. You must remove any personally identifiable information from posted Content if the user has not filed a form. If you request personally identifiable information on your baord, you must specifically state it excludes users under 13 years old. If you become aware of a user under 13 years old that has not entered an accurate birthday, you must have them update their birthday in their profile. You are required to delete any users who mention that they are under 13 years old if you have opted to ban user under 13 years old from registering.
Source
What you're referring to is likely the Children's Online Privacy Protection Act (COPPA), which is commonly mistaken to mean people younger than thirteen years-old aren't allowed on websites due to their content. In reality, it's an act meant to prevent them from giving out their personal information online.
COPPA Website
I used to think the same thing, as well, until I reviewed it awhile back. =p