Speculative fiction wasn't really a term i was familiar with in my early years of reading. In my earliest years, I was reading Dr. Seuss and the Bernstein Bears, and looking at the Time-Life series of science books and developing a strong sense of wonder.
When I'd transitioned to more "mature" reading, the first "series" that I really followed was the Hardy Boys series, along with the Three Investigators. I also read my early science fiction books like "The Girl With Silver Eyes", an early Young Adult Science Fiction novel about a young girl who discovers she has telekinetic abilities as a result of her mother's participation in an experiment, and the more difficult (at the time) read which was Dune, by Frank Herbert.
The latter book came from my Dad's collection, as did the Lord of the Rings trilogy, which I was only able to complete half way through the second book.
Other fiction came from the grafiction arena: Tintin, Asterix & Obelix, and DC & Marvel comics.
I often go back to these books and try to remember my early experiences - what turned me off, and what drew me further into a story - when I try to write my own fiction now.