OP - first of all, welcome to FT!
To your point about being told that the economy check-in line is over there... Agents have never been that rude to me, although I don't doubt your story. In fact, over-entitled passengers have been worse. I politely tell the agents that I'm *G/Elite/Business Class/etc. and they let me go on without any trouble. Sometimes, they'll even offer up an apology. I've seen them mostly ask your status/class of travel before you get in line, which is fine with me since they are enforcing the priority line.
As for the nosy over entitled passengers, I thank them for letting me know that I'm in the correct line.
I find age-ism and class-ism a lot worse in Asia. You definitely need to be dressed or look the part in order to stand in the first/business/*G lines. I've had agents question me and not an older person, even if the older person is the one actually in the wrong line.