problem change type of field DateOfBirth

Hello everybody, I have a problem with field DateOfBirth, I changed the type of field Date to text, when I try register new user on the join now page, I receive the follow message :

You cannot join the site if you are younger than 18 years

That message show me, with any  date that I enter on this page.

Someone, can help me please?

Thanks.

Mysql and Dolphin expect the Date of Birth field to be of type Date. Text is not compatable with the field type expected.

 Thanks for the answer, but I need to change the type field Dateofbirth, how can I do that? Because the type of controll that show on quick search is a scroll, and I need a text field or another type of controll.

Can you help me about that?

Thanks.