I was an odd kid. My parents had lots of SF books around, and I loved the covers, so I decided to start delving into them, when I was in third grade or so. But first, I did some research. Odd kid, remember? I figured that if I wanted to get into this whole science fiction genre, I should start at the beginning. So I looked for the oldest SF books I could find, at home or in the library. Jules Vern. HG Wells. Great stuff.
I worked my way on up. Asimov, Jack Vance, AE Van Vogt. Bradbury was a particular favorite, and he used to come speak at my local southern california library every year or so, making him extra special (and giving me a large collection of signed books). By the time I got to Heinlein's juveniles, I was reading well above that level, and Time Enough for Love became a particular favorite, when I was in sixth grade or so. Of course, I was also reading lots of fantasy - Lord of the Rings, Elric, Fafard & the Grey Mouser. I read a lot.
And sure, I'd been exposed to science fiction before I really started reading it, like Star Trek on TV (Star Wars didn't come out until I was already reading the stuff). But in retrospect, my real appreciation for the genre came from The Twilight Zone. Star Trek may have space ships, but Twilight Zone was real science fiction, as far as I'm concerned...