In the late 90's to early 2000's, I considered "retro" games to be anything from the beginning of time right up the the NES. So, roughly anything that fell out of popularity about 10 years in the past. It didn't seem weird because in the late 90's, we were calling everything in the '80's retro. All that funky synth music and hair was pretty weird and alien to those who grew up on either side of it, after all.

The most modern gaming system in our living room (if you don't count the Steam Deck) is a Wii which is legitimately a retro system these days and I get a little grumpy every time I am reminded of this.