DS (age 19) and I picked up my mum (mid seventies) yesterday and then drove to his gf's house. He got out of the car and his gf came out to say hello (first time she'd met DM).
DS said, "GF, meet my Gran". We all chatted for a bit and I then drove off with just DM for our day out.
My mum rang me later and asked if, as DS was now adult, he'd prefer to introduce her by her name, not as Gran in case it sounded childish.
DH, DS and I thought that was a really odd suggestion!
YABU he's adult and you call grandparents by their first name
YANBU nope, still Gran / Grandpa / Nana / Grandad / Nanny / Grandfather/ Grandma etc here whatever the kids' age!