I have a good friend in his late 60s, and I've made an interesting observation that absolutely no one in our social circle will refer to the female he is (indisputably) dating with the term "girlfriend". A frequent comical substitution is "lady friend".
I wonder what it is about the word that makes people not want to use it in reference old people. I would think that term goes hand in hand with "dating" and would imply exclusivity, romantic interactions, possible courtship to marriage, and maybe sex. In the case of my older friend, no one appreciably doubts any of these points (oh dear have I said too much), so what causes people to not use it?
There has been a similar question What is a synonym for "girlfriend"?, which really just strengthens my case. The difference is that I'm asking why someone would want to avoid this, whereas that question is asking how to avoid it.
Question: What about the history, definition, or nuances of the girlfriend-boyfriend relationship terminology causes people to not want to use it regarding old people?
I get the feeling from some comments that I need to express what my own use would be. I have generally associated "dating" with "girlfriend"/"boyfriend", and considered them to be the most basic and direct way to refer to those things. As such, I would find applying this terminology to any unmarried couple to be preferable. I believe that the hesitation I refer to in the title applies to all demographics.