Where it says D.O.B. (in edit profile) put your full D.O.B. as viewable, then it adds a pretty ickle cake and your age. I think that if you put day and month only as viewable, it keeps your age a "secret", then we all will think you are 61 and ashamed that you always made out you were 22 !!
Yup just tried it, if you only set day and month of birth and "show" it keeps your age secret.