If you were in a work environment and your boss referred to you as a girl in a formal business meeting would you be okay with that?
The old one of referring to a shop assistant as a girl, even if she is in her 50's or 60's, creates an impression that the man or men working in the shop must be more important. After all, how could a girl be in charge of a man?
If you wouldn't refer to a female judge as a girl then you shouldn't refer to a female shop assistant as a girl.
What friends call each other in a social setting is of course a totally different matter.