KatyK I'd also be upset and probably mention to my daughter (in private of course) to please include me by introducing me. I can't really comment because my daughter always introduces me to people as "This is my mum" and then gives my first name. I doubt it's the old and invisible thing. Perhaps it is just being a bit thoughtless. Say something, gently.
A while ago we were at a fairly distant family member's house at Christmas, and there was a really elderly woman there (in her 90s), someone mum, and my daughter came to me during the course of the afternoon and said that she felt sorry for "that old lady" because people were ignoring her "and so I'm going to take her around now and introduce her to everyone, make sure you talk to her about Eastenders" . And I never watch it!
I read this thread thinking it would be about the dreaded thing that happened to me recently - when someone came and joined in where I was talking to another person - and I couldn't remember the person who had joined the conversation's name - and I knew that the longer the time ticked by, the ruder I would seem, especially as he had greeted me by name, and I should have known who he was! (In my defense, I meet a heck of a lot of people through work). Anyway I was saved by the bell because a fourth person joined our group and greeted the "joiner" by name, which jogged my memory for his surname (phew!).