Linda Sue......"Grammie," LOL....I don't know that I feel what someone calls their mother is related to their age (or age group). It's a really personal and Familial thing. It really depends on how it all got started. It's also more related to the type of relationship one has with their Mom, IMHO.
This is an interesting question, Sunshine.....making me think back at what my sister and I called our mother. If you can believe this or not, I recall we used a couple of renditions....as in "Ma" and sometimes "Mother"......They both have explanations.
Our mother and her siblings called their Italian mother "Mama".......and we obviously shortened that to "Ma," for our mother. Sounds a bit informal, but we were an informal family...so.....
When about to approach a "serious" matter...we would say, "Mother.." as if to warn her that the conversation would be something important.
I do have an adult friend who calls her mother, "Mommy." It sounded odd to me at first...but after getting to know the family, it became clear to me and I understood it was not a "juvenile" thing.
Now....I don't know about all mothers, GF...but what I want to know is, what do my sons call me ....when I'm I'm not around!! LOL!