Developing a social site (stories and stuff) we need a system to ensure general safety usage for minors. Since there are Child Online Protection Act and anything else in place, we need a reliable system to ensure that no child accidently enters those areas.

Since a simply adding a select your birth date or age would not make it, I guess. Entering credit card numbers is a boomer (not much user will do this). If you come along with a facebook login or google, is it enough to test the birth day date within the profile?

[Update]

Since it is difficult to talk about different countries lets limit the discussion on USA, UK, Germany and maybe Brasil and France/Spain/Itally. What systems are used by sites originating in those countries. Any information would help. 

Often one sees Whats your Age, Are you really 18 years or older? YES / NO. Is this really enough?