I think the game-spoiler business has very little to do with how old a game is.
I think it deals more with the level of considerateness a person has or doesn't have.
Therefore, it doesn't matter matter how old a game is.
An inconsiderate person can make up his/her own reasoning as to why he/she is blatantly spoiling something.
You know these people.
The "oh I just saw said movie in theaters, so let me spoil it by telling you what I liked/disliked."
You see it with got, "oh the books came out years ago, so I can talk about anything even though the show isn't up to date with the book."