Back in the PS2/Xbox/Gamecube era the SNES and Megadrive seemed to be retro while the PS1 and N64 were just “old”. So maybe 2 generations ago is the start of retro.
I think it’s definitely a lot blurrier now though. The differences between consoles and the leaps between generations are less pronounced, and there are so many y rereleases and remasters now keeping older games fresh.
Also works on the GameCube, albeit with much more limited IO due to the lack of USB.
The GameCube Can Now Run Windows
I’m not sure there’s much fun software compiled for PowerPC Windows NT to run on it though (yet).