I'm in my mid 40s and dating this lady of a similar vintage. I am
trying to find a good word to describe our relationship, but
"girlfriend" and "boyfriend" seems inappropriate for us.
I find myself in a similar situation.
What follows is my very subjective and personal viewpoint. No doubt there will be some who will disagree with my interpretations, but I hope it explains why I (along with many other middle-aged people) share this dilemma.
I am 48 years old while my boyfriend, an Englishman, is only 57. It does sound a bit awkward when I talk about my boyfriend with friends and relatives, as neither of us are in our teens or twenties and for better or worse we normally associate girl and boy with young people.
I feel uncomfortable saying partner because we do not live together, and that term (for me) expresses a long-term commitment. I would use, partner, when you have been living together for a certain time, and the relationship is consolidated. As an expression, partner, is acceptable at any age, it's the unmarried equivalent of spouse.
I don't think my boyfriend has ever called me his Lady friend to anyone, and I have never heard him use that term within earshot. It lacks intimacy, and sounds old-fashioned, if he were to ever use that expression it would be in a humorous, jocular manner.
I have never been called a mistress in my life by any boyfriend, even when that man was married to someone else. But technically speaking, in an adulterous affair, the term, mistress, is appropriate. For this reason my current boyfriend (who is divorced) never calls me this, and I would object strongly if he ever did.
The term, significant other, I hate. I couldn't possibly bring myself to call any boyfriend that. It sounds horrible, and what does it mean? "This is the other person who plays a significant role in my life"! :)
However there is a term, which we call each other with affection and it works for us. Sweetie. You're my sweetie is something we say to each other, and with very close friends it's a term that doesn't sound too sugary or cloying. I like being called sweetie. The term, girlfriend, is slowly growing on me but will I feel the same in my mid-fifties? I doubt it, unless of course he moves in, in which case I'd be happy to call him my companion or partner to anyone.