[user104658]

It's not acceptable for obvious reasons. On the other hand, if it's being used as an identifier in a private conversation between two people out of earshot of anyone else, then I don't think it particularly matters.

E.g. I have two young new-starts at the moment. Also in my shop there are two women, one is 62 the other just turned 40. If I was working with one of the young guys and he did something incorrectly and said that one of them told him to do it that way, and he couldn't remember their names and said "The older one"... I wouldn't think that was a problem.

... If the next day he then started referring to her as "the old one" to her face, that WOULD be a problem.