I was 4 years old when we moved to Philly. The corner grocery had a box of cover-free used comic books. I had a choice of spending my candy money on comics or bubble gum. The comics lasted longer. Some of the stories are still with me. It helped that a lot of them were LEGION of SUPER-Heroes from the Edmund Hamilton and Otto Binder era.
Once I got hooked, I read everything I got close to. Fiction, non-fiction, references, news. World Almanacs (cover to cover), encyclopedias, and old books wherever I found them. Not always at the library. One of the neighborhood kids had old LONE RANGER books (text, not comics) at which point I caught on: TV and movies came from books. From there it was just a jump to Jules Verne and other adventure classics.
By the time I was 14 I discovered the Science Fiction Book club and got a quick education in the genre (courtesy of the Science Fiction Hall of fame Volume 1, Foundation Trilogy, and Stranger in a Strange land) followed by Princess Of Mars and the entire ERB paperback catalog from ACE and Ballantine. Some summers I racked up two books in one day.
Looking back, I was just lucky to run into the right material at the right age.
Anyway, in my case at least I know the afinity is genetic and comes from both sides of the family. When I first visited a paternal uncle I'd never met (in my 20's) I found a loaded bookshelf full of books I was familiar with. My siblings read, my mother reads non-stop, and most of my cousins. Doesn't mean everybody reads SF but the smarter ones do.