You can tell the age of a tree by counting its rings. You can tell the age of a person by counting the number of times they say something anachronistic. For example, if I start talking about Compact Discs, kids will instantly identify me as being, uh, mature. They will make snide remarks about the La Brea Tar Pits and mastodons. Kids can be cruel. Ask me how I know.
Original Resource is Sound & Vision