User Birthday Module
Using Jamroom
After installing and activating the Birthday module, you'll get new form fields in the user account settings to add the Birth Date for the user, if that is not set, by the user, then it will not show on the profile page. You may even need to add the variable to the profile templates as I don't think it is on every skin. The jrFollowMe skin has it but not the rest of the skins as far as I'm aware of.
Hope this helps!