What spoilers spoil are the artist's intention of how the story should be revealed and the audience's initial experience of the art. Because I respect both of those things, I don't care how old something is—I won't spoil it. If you don't know what Oedipus Rex is about, read or watch it before someone rushes in to tell you.
The thing I hate most about people who spoil is their attitude. I'm not sure which are worse, the ones who're gleeful or the ones who're indifferent, but neither think it's wrong to deprive someone of an experience.
That said, I believe in consensual activities, so if a spoiler and a person who likes to be spoiled find each other, I'm happy for both of them.