Old is when you're still complaining about Firefly being cancelled and realize it was over a decade ago.

Old is making a Star Wars Phantom Menace reference and someone telling you they were too young to remember watching it.
Old is realizing that CD-ROMs were cutting edge tech when Shadowrun debuted, and wireless phones and several devices from pwniexpress would have solved several problems in old adventures for the setting.

Old is showing someone your vintage Luke Skywalker Kenner action figure, and being asked, "who's the blond guy with the blue stick in his arm?"
Old is remembering when the majority of internet traffic was as ephemeral as a college bulletin board. (and remembering that a bulletin board consisted of cork, not software.)
Old is being able to come up with this list.
*******
More seriously, Old is seeing the story on Jim Ward, and thinking about Gary, and Dave, and Tramp, and Nigel Findley, and Aaron Allston, and Jack Vance, and realizing that the pool of the original old guard I used to lionize as a young kid is constantly shrinking.