Some answers to this are easy: NES/Famicom, PS1, Game & Watch, etc. However, as time rolls forward, the lines between getting older and retro are becoming more blurred for a lot of video game consoles.
Is there a time period that must pass for a video game system to be considered retro? Is the Gamecube considered retro now? Wii (9 years old)? Xbox? PS2? How do we define retro video game consoles?