I often use the phrase 'It's been ages'. I recently saw someone say, "you've been ages", and I'm curious if it is grammatically fine.
They mean different things:
"It's been ages" is sometimes a form of greeting, when you haven't seen the person in a long time, or used when you haven't done something in a while.
I need to go to the gym. It's been ages since my last visit.
Sally, hi! It's me, Julia. Wow, it's been ages.
"You've been ages" means the person has taken a long time to do something and has probably kept you waiting around.
There you are, Mark. You've been ages. You left for the bathroom over two hours ago.
These phrases are somewhat common in British English, and technically they're grammatically correct, but the use of them is informal and as such, should not be used in formal writing like academic assignments.