When you take your (ex)wife and stepbrother to a Poison/Ratt/LA Guns & Great White concert and they have NO IDEA who they are seeing.
Or more recently - make a Tron movie reference to your co-workers and they are all too young to have seen it
or when da1337nesMonstr doesn't know who Roy Orbison is edit: When most of the people on this site don't know who Roy Orbison is....
He's the guy who was tattooed on the coaches butt in Waterboy right?