The science fiction of Asimov & Heinlein at 11-13 (just open the mind to speculation) come to mind, but there were also the books I found in my grandmothers garage that had belonged to my father and uncles..Jerry Todd books, The Speedwell Boys..etc.. also "THe Wonder Book of Myths & Legends"...all the Greek & Roman heros for the 10 year old mind! Wonderful!

Then...at age 17, working as a grocery store checker on a slow Sunday afternoon, I picked up "The Age of Ideology" in paperback from the book rack...one of a series of introductions to Philosophical thought, Boom! hooked!...I even managed to sneak in a book report on Nietzsche's "Zarathustra" in American History class..*grin*

I will confess to having devoured ALL the Heinlein and Robert Rimmer 'social experiment' stuff in the 60s & 70s, and though I was well aware they were not great literature, they were important ideas. Rimmer used to provide lists of suggested further reading in the back pages that would keep one going for years.

Also...an interesting way of looking at the question....I once acquired a copy of Oswald Spengler's "Decline of the West" when I was 17-18...and didn't understand a word of it! The experience of finding something entirely too dense for my head to wrap around was good for me, and later, when I encountered "Chaos Theory" and other arcane concepts, it helped to have perspective on just what sort of things WERE suited to my own thought processes.
(I found I could NOT finish John Barth's "Giles Goat Boy", as much praise as it got from some...nor could I cope with much "sword & sorcery" sci-fi....but I could roar thru 'hard' sci-fi like "Ringworld" and "The Mote in God's Eye" at a blistering pace.

Fascinating thread....