BirthDate feature not working correctly

Right now I have dolphin configured so that you have to be at least 18 years of age to join. But somehow people get away with not putting in a birth date. For instance, while logged in as admin i noticed an account that got created. The birth date info was something like 00/00/00.

Is there a way to make is so that dolphin notifies the user if their birth date reflects that they are under 18 and also have it so that the profile does not even get created?

On the profile fields. DOB, under advanced there is a box to check values, you can place a php expression to check for the values

similar to the email, also make sure you made the field "mandatory"

I have no idea when it comes to PHP. Could you point me to a place that shows how to make the expression? I could probably figure it out if I have something to go by.