There is no way to do it using their birthdate, but I can give my kid an account under my birth date, using their name, their pictures, their info and allow them use it, according to their rules.
Its all them protecting themselves, they don't actually care who is behind the computer screen, they just don't want to get sued. It is against the law for *any* website to collect information about a minor, but yet any picture or piece of information pertaining to your children that you upload to the internet (blogs, photo albums etc....) is breaking this law. Its just too arbitrary to enforce. That is the only reason that rule is in place for FB.
I didn't lie when I created that account for my son and nobody can convince me I did, I stand by that 100%.