zahc,
You can't expect everyone to keep silent about everything that happened before you were born. That's ridiculous. In essence, you're insisting no one in the world talk publically about any movie, television show or concert on the off chance someone, anyone, younger than they standing nearby hadn't seen it yet?
One of the down sides of coming along after things happen is everyone else knows about them but you. Wrath of Kahn has been the subject of as much pop culture reference as any movie ever. Hell, the next movie was titled "The Search for Spock". Spock's death in the film was unexpected if you weren't a "fan" and read reviews and promo material and such but wasn't a "secret" even when the movie came out.
Argument from history, not age, is perfectly valid. Should no one "spoil" the fact the Berlin Wall came down? Or that Challenger exploded on takeoff? Hey, guess what, the Eagles got back together for a tour in the 90's. Things happened before you were born, other people want to talk about them, get over it.
"spoiler warnings" properly only exist in two circumstances.
1) the film is still in theatres during its first run - in this case it is reasonable to expect someone who might want to see it hasn't gotten there yet.
2) the film is fairly recent and absolutely hinges on a key plot point, like the Crying Game or Sixth Sense, even then, once it's been on video for a while it becomes a bit whiney to expect everyone else to go out of their way in not discussing it.
Wrath of Kahn is a quarter of a century old and yet all the ST fans of all ages have seen it. You've had a decade to get around to it yourself.