ProBoards asks for your birthday when you register. There is no requirement that you enter the correct one. If you did, you should check your profile and see whether or not you are sharing it with other forum members. When you edit your profile, go to Privacy and scroll down to the birthday information. You have the option of hiding it, showing just day and month, or showing your complete birthdate.
As best I can tell, if you hide your birthday information it hides it from administrators as well.