A discussion of the security trade-off:

Child-safety activists charge that some of the age-verification firms want to help Internet companies tailor ads for children. They say these firms are substituting one exaggerated threat -- the menace of online sex predators -- with a far more pervasive danger from online marketers like junk food and toy companies that will rush to advertise to children if they are told revealing details about the users.

It's an old story: protecting against the rare and spectacular by making yourself more vulnerable to the common and pedestrian.

http://www.schneier.com/blog/archives/2008/11/online_age_veri.html