It's just an informal expression meaning "in a very long time.
Come on, let's go to lunch. We haven't been out together for lunch in ages.
Oh my gosh - I haven't seen you in ages!
Also, you can use "it's been ages since."
Mom, it's been ages since you said we could go to the park. When are we leaving? (Translation for a 7 year old: at least 15 minutes)
It's been ages since we've been to this restaurant - look what's happened to the prices.
Does that help?