Perhaps Grandkids are 18 & 19 and very immature regardless of their age would NOT be a Grandup.

My Grandups are so mature for their age, Maybe they have been emancipated minors for 4 or 5 years,
Young but maybe living on their own,have jobs, maybe good credit scores. Some able to pay their own way.
He's my Grandup and just turned 20 Friday.
I have 5 grandkids 2 being Grandups.

You're all stupid people and I hate you.