20+ years seems like a solid cutoff point.

I would say anything before circa ~2000 is definitely retro.

It does get more complicated with the release platform.

PC games can be updated and can even take a life of their own, while console games tend to be tied to a specific console/generation and the associated gameplay experience.

One good example is Simcity 4, even before decomp/recomp's got big, the community modded it to such an extent that it wasn't really the same game as the one released in 2003. Simcity 2000 is definitely a retro game though with Simcity 3000 being somewhere in between. Recent decomp/recomp have added much needed modding capabilities to Simcity 3000, but a lot less people play 3000 versus 4 (and the modding scene is a lot smaller).