The first grown-up novel I ever read was Stephen King’s Carrie, at eight years old. There were always lots of horror/thriller novels in my house, and Carrie was, for whatever reason, the one I picked up. I didn’t comprehend all the stuff about menstruation, but telekinetic destruction was quite marvelous, and from then, I was hooked. My great-grandmother had a spare bedroom full of science fiction paperbacks; one of my great-aunts had Clive Barker’s Books of Blood; the local library had the first couple volumes of The Year’s Best Fantasy and Horror. I read ’em all.
Watching The Twilight Zone probably had a lot to do with it, too.