At least in it's present state you can't leave any of the feilds blank it requires them all might be a program setting though maybe Paul can adjust I don't know as I don't have this on my board yet. So until then only option is to reset the yearly date to keep at your desired age.
Just a quick note as some of you already know Pauls' been updating the forums software and one of the new features is a birthday Calender which is done by your profiles which now has a birthdate entry area. To find this new feature it can be found just below the users online section of the forums main page. Have a look it should show the December calender and you'll see how it appears when someones birthday pops up.