When people started to call me that. When i grew up i first met games at a friends house where he had a SNES with the Hulk game (can't remember which one) and i used to play there. Later, at another friends house where they had the N64 and i used to go play goldeneye multiplayer. And i noticed they sometimes didn't want to play and i was confused, because surely there was nothing more fun than actualy playing games was there?
And then my dad got a laptop that could run duke nukem, doom and prince of persia and things like that. Got confused as hell again, because i'd try to show it to friends and after half an hour they'd want to go do stuff like riding bikes, pretending to be pokemon or any of the regular kids' stuff. Pokemon on the gameboy also deserves honourable mention since i was one of the only 4 people i knew to finish that game. Ofcourse this doomed my pokedex to being short a couple of entries. *edit* people i knew in my small circle of friends in a small school when i was 10
Same again with the first playstation and when i finally got my ps2, most of the people i'd play growing up with, stopped playing. So they called me a gamer, as an insult sometimes, but mostly because people need everything defined around them.
And now i've found myself years later owning a total of 8 gaming platforms and 255 games. So yeah, i didn't know i was a gamer untill people told me i was. Otherwise i'd still not know, because i only went to look for friends who'd react to the same 'insult' as i did.