(Serious):
If this is repeated anywhere, apologies.
I didn't read every page of this. So for me, "retro" for video games, at least refers to when the console itself has reached end of life plus 5 years.
I see that as the time when things should be considered retro, at least with video games.
Movies, on the other hand, I think a decade makes a film retro.
With Books someone else said 100 years, but I would say more around 40-50 years. Things change in the world so fast now, and with how language, political views, and opinions change so quickly, retro isn't as old, at least how I see it as it once was.