I didn't read all the responses, but I would:
A) Tell her that the time clock is not the law and any professional who gives a shit about their job does a little extra work when necessary with no complaint. Be straight with her and tell her she's being unprofessional.
no results? B) Rat on her.
I know how you feel... I'm the youngest person in my office by a fair margin. At first it was really uncomfortable and I wouldn't dream of confronting anyone. Now I realize that it doesn't make a difference and we are nothing more than our role in the office... age doesn't matter. Not that I have to confront people all that often, but ya know
:edit: All of my receptionists are the bomb. Very smart and professional and never bitch about being asked to do more. We have about 6 of them, and we've had a steady stream of new ones over the last 4 years I've worked there. You should have no problem finding a good receptionist.