My parents don't allow me to shave moustache.

They say you can only shave your moustache when your father dies. I am 18 and shave my beard since it is patchy for now and I would prefer to be clean rather than have weird hair until I can grow full beard. They also say I will look older (making them look even older) after I get darker moustache if I shave it. I haven't seen my father without a moustache neither my older brother. What should I do? I can rebel and shave it but it scares me alot since my father is a heart patient.