Retro is like oldies.
You can't just go by decades to determine if something is an oldie, just like you can't go by console generations to determine if something is Retro.
For instance, the NES and Gameboy had a much longer life span than games have now. So when calculating retro status you must include that in the equation. Also, you must include what type of games are being made and the depth.
For me, all the classic 2D consoles are Retro, and only the first generation 3D consoles are Retro now. So that means we are 3 console generations out for 3D systems. Nintendo 64, Playstation, and Saturn are 3D retro systems.
I will add the Gamecube, Playstation 2 and Xbox systems next generation though honestly that seems too early even. So when PS5 and Xbox 4 comes out, I think we can consider looking at the second generation of 3D games are Retro.
Afterall, I would not consider music from the 80s and 90s as classic oldies. Now that I think about it, the thing that really pushes the oldies music and retro gaming status is the shift from 2D games to 3D games and the the changes in Rock and Alternative system of music. Those changes really pushed the recognition of a change in gaming and thus Retro was born.