My theory is that nostalgia requires considerable time away from a thing but not so much so that the person forgets about it - plus positive emotions. I believe that the minimal "time distance" is five years though it seems for most people (or at least the companies that look to cash in on nostalgia) that the average time distance is ten years.
Yet it also is not required for a thing to be old, just for you to have watched or played it a long time ago. For example, a game could be ten, twenty, or thirty years old, but if you play it every month or talk or write about it each month or so, then you might lose your nostalgia for it as the game "never really goes away."
Furthermore, a game that you played last year for the first time is probably not going to elicit nostalgia a year later unless it looks like or sounds like or reminds you of something else from the past that already makes you nostalgic. For example, if you played an Atari or NES game this year that you never heard about before, then thinking about it a few months later probably won't give you nostalgia for the game.
However, a "new old game" might bring forth nostalgia for the other games for that console which you played when you were younger or just the era during which you played it even if that time was ten to twenty years after the introduction of the console. For example, someone playing Atari VCS / 2600 games during the 1990's might go on a nostalgic trip thanks to seeing a new homebrew game that reminds him of the 90's even though that's not when the VCS was new.
Additionally, positive emotions are required for the nostalgia to exist. If you were indifferent about the object or time, then you won't be nostalgic thanks to it. If it was a bad time in your life or the game was bad, then nostalgia probably won't happen. Yet if that game was fantastic or if it was your escape from your bad times way-back-when, then it could cause you to look at the game - or the good times which it gave to you during those bad times - with rose-tinted glasses of nostalgia.
So what do you believe? When does a game, a console, a controller, a toy, a cartoon, or any thing, go from "modern" to "nostalgic?"