Star Wars blew my mind when I first watched it in the late 80's so I can't imagine the impact it would have had seeing it as it was released in theaters. I already knew Vader was his father, for example, just from people repeatedly quoting that line and I had also seen most of the SPFX shots here and there on TV. But to go into it not knowing what to expect must have been an altogether different experience.
I have a friend who has never watched any of the SW movies (and he's 28 years old!), but has a pretty good grasp on the plot and characters just from things floating around in pop culture.