What is classed as retro?

At what point do we class a game as retro? 10 years old? 20 years old? Longer?

A game is categorised as retro if it's older than say 15-20 years. PS2 and Xbox 360 games can be categorized as retro now. I know a few people who are still playing PS1 and PS2 games, just for the sake of nostalgia.

Anything that isn't a modern game—or one from the very recent past—can be considered "retro." It’s hard to say exactly how many years it takes for something to qualify as retro, but anything over 10 years old could certainly fall into that category. Generally speaking, "retro" refers to games from the Super NES, Sega, Game Boy, Game Boy Advance, and PlayStation 1 eras; even the PlayStation Portable (PSP), PS2, and Nintendo DS (NDS) could be considered retro—if they aren't already.