Yeah, it's definitely not as simple as "film came out 5/10/20 years ago, therefore spoilers are fair game". But if you are talking about a film like TESB or even something more recent like TDK, films which everyone and their mother has seen - and if they haven't, shame on them - then I think spoiler tags are a waste of time. I would probably still spoiler tag something like The Usual Suspects though, simply because that's a film that hangs almost entirely on a single plot twist and knowing it in advance really would ruin the film.
However, most films are more about the story than the plot so knowing the ending doesn't really matter. I find it bizarre when I hear people say they aren't going to bother watching a film now that they know the ending. Why do we watch certain films over and over again? It certainly isn't for the plot because we already know what's going to happen; it's for the story and the characters.