First example coming to mind: I was watching The Maltese Falcon a month or two back, and at one point Humphrey Bogart's character tells his (female, just to be clear) secretary/sidekick: "You're the man, sister". That movie is from 1941 - I certainly wasn't expecting lines like that (nor the equally great "When you get slapped, you'll take it and you'll like it".

Of course, it depends on the movie - I started watching Fire Over England not long after that, because it has Vivien Leigh in it, and didn't get very far in that one, in the first fifteen or so minutes at least it never even came close to That Hamilton Woman, Gone with the Wind or A Streetcar Named Desire. Those three are all excellent and entertaining films, though - and they are from the forties, thirties and fifties respectively.

Since Casablanca (haven't seen it either) is actually more recent than Citizen Kane, Gone with the Wind and The Maltese Falcon, I think it's safe to say you can't draw any conclusions about it being boring just by looking at its age.