Spoilers are never OK, no matter how old the subject content is, unless the people who hear it want to hear it.
It cannot be assumed that someone has seen or played something simply because it is old. No matter how old something is, there is no assurance that anyone has seen it. People do not see all classic or popular movies, because they’re not forced to, and there is no way to get everyone to see them.
You should not expect people to know it
There are so many movies and games that it is impossible to watch them all. Even if you only take popular movies, it would take a lot of time out of people lives. It is unreasonable to expect everyone to spend that much time on media. If you think that there are certain movies that everyone must see, your list is likely not objective and differs from other people’s opinions of what people should see.
“If you were going to play it you already would have.”
This statement is arrogant because do not know if they would have played it. If they haven’t heard of it until recently, or if they didn’t realise how it was or didn’t realise that they might like it, then it is entirely possible that they would not have seen it. There are a lot of films that people have heard of but haven’t seen. The mere fact that people have limited time means that people will generally not see every film that they hear of. Especially with older media it becomes hard to find or buy them, and if you can you need to make sure that it is worth the money. Even if it is free, you need to justify spending time on it. Not every film that other people like is necessarily something you will like or even not regret seeing.
“If you haven’t played it by now then you’re never going to play it”
No one can predict what others will do in the future nor can they know someone well enough to know what they will and will not watch. If you think you can then you are being arrogant.
If someone has just got into gaming for example, there are millions of games that they have yet to play. If someone only watches films every once in a while, for whatever reason, you don’t know what movies they will see. Not everyone watches only new movies, a lot of people watch older films at arbitrary points in their lives based on a variety of things.
You do more harm spoiling something than the benefit of talking about it
Most conversations can do just fine without spoiling. If the person you’re talking to doesn’t want to hear the spoilers, it is easy and simple to not spoil it and talk about something else. There is no need to talk about spoilers most of the time, and you probably won’t lose much by not talking about it.
If you and someone else agree to talk about spoilers, but someone else in the conversation doesn’t want to hear them, just talk about the spoilers when that person is no longer in the conversation.
It’s not hard and it makes sense, spoiling stories and gameplay might ruin a game for someone, so don’t do it.