Google's page for creating a new account is famous for only requiring your email address and your country, so it's strange to see that users from the United States have to enter their birthdays.
Google's terms of service say that "you may not use the Services and may not accept the Terms if you are not of legal age to form a binding contract with Google". If you try to enter a date like 4/28/2009, Google shows this message:
"In order to have a Google Account, you must meet certain age requirements. To learn more about online child safety, visit the Federal Trade Commission's website."
{ Thanks, Itamar. }