How old does a game need to be to warrant a remake?

[Mnementh]

I don't think you can put a proper number of years on to that, thta is decided by other factors that result in faster or slower "aging" of a title.

From the viewpoint of management, a remaster or remake is warranted, once it is conceivable, that it will sell well enough. Maybe new demographics entered gaming that never touched the game in the first place or the original is somewhat unavailable and therefore a remake might sell well.

For gamers that played the original game, that line might be different. Maybe they want all their games on their newest hardware. Backwards compatibility is one way for that, but remakes or remasters are another. But the major point are probably enhancements that improve the gaming experience. That can include, but is not limited, to graphics. For graphics we have good points what is still acceptable in old games and for what we like new stuff. For instance if the original wasn't HD, then a HD or 4K upgrade will do nicely. But there might be other things. Maybe the game had bugs or glitches that a remake tries to remove. Maybe the remake can add a mode, that can improve the original game. Or maybe simply the original game had great multiplayer, but the servers are no longer working and the remake brigns new servers.

So many things could define a remake worthy. I don't think it is easy to pinpoint it to something specific.