20 years seems like a good benchmark. Only thing is, I can remember back in 2003, when I first got the retro game bug, and started playing the Super Nintendo and Genesis again. I thought those systems were "retro" even though the Super Nintendo was only 13 years old by then. Maybe it was because even though it was only a mere 13 years, it had been several generations, and generations with dramatic technological shifts. Comparing a Super Nintendo game to the most advanced Xbox game in 2003, was a gigantic difference in technology.
The Dreamcast is currently 17 years old, yet it doesn't feel as retro to me right now, as the SNES did back in 2003.
I think maybe the technological leaps between whatever generation you're considering and the current modern day generation, is a huge factor.