I certainly am old enough (39) to remember it at the theater.
A bunch of us lined up at the local mulitplex after school for the 7 PM opening show and discussed the rumours about Vader being Luke's dad while waiting.
Most of us were dismissive, but one guy kept saying it was probably true.
My reaction was 'Ohmygod! Vader is Luke's father.'.
"You say that it is your custom to burn widows. Very well. We also have a custom: when men burn a woman alive, we tie a rope around their necks and we hang them. Build your funeral pyre; beside it, my carpenters will build a gallows. You may follow your custom. And then we will follow ours."- General Sir Charles Napier
I was one of the people who saw the first airings in theatres. It gave me chills to realize what those words meant, and it was all kids could talk about for months.
Nitram, slightly high on cough syrup: Do you know you're beautiful?
Me: Nope, that's why I have you around to tell me.
Nitram: You -are- beautiful. Anyone tries to tell you otherwise kill them.
"A life is like a garden. Perfect moments can be had, but not preserved, except in memory. LLAP" -- Leonard Nimoy, last Tweet
I was around 10 and my mother bought me the trilogy on tapes (original versions, still have them ).
I sat down and watched ESB first, having seen SW about a year before. If I had been exposed to the plot twist before then I didn't remember so it came as a shock that Darth Vader, the big bad guy in the black suit was the hero's father. Suddenly everything that happened in ESB made sense to me and it became my favorite SW movie to this day.
"I do not understand why everything in this script must inevitably explode."~Teal'c
I honestly can't remember any one moment where I suddenly realized the revelation and was shocked. I somehow just knew for some reason, even though I was only exposed to Star Wars when I was about ten, and specifically found someone who had the movies so I could watch them.
I think I'd read the comics before I saw the actual movie, so I wasn't surprised, but the scene was powerful nonetheless.
Edi
Warwolf Urban Combat Specialist
Why is it so goddamned hard to get little assholes like you to admit it when you fuck up? Is it pride? What gives you the right to have any pride?
–Darth Wong to vivftp
GOP message? Why don't they just come out of the closet: FASCISTS R' US –Patrick Degan
The GOP has a problem with anyone coming out of the closet. –18-till-I-die
My reaction was pretty much the same as Luke's. I was born in 76' so I was five when I saw it. I thought Vader was lying, because I refused to believe that Luke, my hero of heroes, was the son of the most evil man in the universe.
MM's Zentraedi Warlord/CF's Original Predacon/JL's Mad Titan
I was born in '81 and was exposed to the movies a few times while I was still young. By the time I was old enough to make sense of the movies, that particular bit was already known to me. So basically I've always taken it for granted.
When I first saw the movie, I think I was too young to understand the import of the scene, and afterwards, I avoided ESB (kissing? Yeuck ...) until last summer. Rewatching it, I first truly felt the scene's power, and it's almost indescribable how I felt as Vader said, "No, Luke ... I am your father."
A Government founded upon justice, and recognizing the equal rights of all men; claiming higher authority for existence, or sanction for its laws, that nature, reason, and the regularly ascertained will of the people; steadily refusing to put its sword and purse in the service of any religious creed or family is a standing offense to most of the Governments of the world, and to some narrow and bigoted people among ourselves.
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