I have no idea what the first SF book I read was, but I do remember writing my own SF in third-grade (during free time, and being all mysterious and hiding it from the teacher), so obviously was introduced at an early age. I remember all the early Heinlein, Andre Norton, Piers Anthony, and anything else I could get my hands on. Unfortunately, none of my kids like SF, so I’ll probably just sell all those old copies.
None of my maternal grandfather’s kids liked SF. However, some of his grandkids love it. So don’t be too quick to sell off the old copies.
I’m pretty sure that my first SF novel was Asimov’s I, Robot. I would have been about 5 or so. It’s not really a novel, more of an anthology, but it lit a fire and my dad’s SF shelf on the bookcase became my hunting grounds.
It’s a really amazing prediction - the kid is embarrassed at getting a call from his parent while he’s waiting on line, so he ends the call by saying that he can’t talk at the moment because he’s in public - and the kid behind him in line saying that he avoids such embarassment by leaving his phone in his bag under such circumstances.
I wish-listed ***Runaway Robot ***at paperbackswap because of this thread - and it came up! I’ll be getting my copy soon, without the exorbitant price!
I’m not 100% sure which was the first SF I ever read, but most likely it was The Keeper of the Isis Light by Monica Hughes. I know it was the first book that I ever stayed up all night to read. Pretty soon after that I read a bunch of that author’s other books (she’s a Canadian author who wrote a lot of SF for Young Adults).
Other early SF reads that I remember were the Tripods series mentioned earlier in the thread, as well as The City Underground by Suzanne Martel.
While I was in elementary school I definitely had a thing for YA post-apocalyptic SF. And I’m still pretty much a sucker for the genre.
I remember my first Sci-Fi novel. I found it rummaging around in the storage bins in my Aunt’s basement. A dusty little paperback, quite dog-eared and the cover was half hanging off. A huge sun on the cover, with a shiny little orb in it’s midst. Inside, there were space needles! Sentient dolphins! Smart chimps! A spaceship that can dive into the freaking SUN!?!
I was 10 years old. David Brin is still my favorite Science Fiction author.
I don’t remember my first SF story. I had a brother 12 years older than me, and by the time I learned to read, he had amassed quite a collection. I remember Runaway Robot, Eleanor Cameron’s “Mushroom Planet” books, and Gordon Dickson’s *Secret Under the Sea *. And a whole lot of Danny Dunn books. And a whole lot of Edgar Rice Burroughs. And Andre Norton.
My dad’s friend sent him a copy of Deathworld Trilogy by Harry Harrison, so that was technically the first science fiction I ever read. It was okay (hey, I was 10)
I went to the library to find more, and while looking through the 'H’s, I noticed Podkayne of Mars. I read it based on the title. I really wanted to be Clark. While Deathworld wasn’t bad, it was Podkayne that really got me hooked.
IIRC, the first SciFi novel I ever read was either Star Ship Troopers or Stranger in a Strange Land (or, possibly the Moon is a Harsh Mistress). It’s been so long that it’s hard to remember now, but I remember being totally captivated by the concepts and vision. Course, now those are all pretty dated I guess to most readers today, but when I read them was just at the dawn of Apollo, when we were still going to the moon, which was pretty cool. 
-XT
I started with ERB’s A Princess of Mars. I bought all of the Mars books out of my allowance in the Ballentine paperbacks at fifty cents each. I still have them.