I guess I was likely fertile ground at age 12. I'd read a lot of fantasy as a kid. I don't think I could pin down the exact book which got me started, but I remember having the young, hip, and pretty teacher (my Miss Landers, every boy has one at some point) read us the Lion, the Witch and the Wardrobe in third grade, a chapter each day. I was a bit blown away and read the whole series and many others. The Hardy Boys and talking animal books were dead to me after that. The next year brought a different teacher, who gave extra credit for Newbery Medal books, which is still one of those things I should have thanked her for at some point before her name slipped out of my memory. I might not remember her name, but I still remember Susan Cooper. Ursula Le Guin, and Lloyd Alexander, although I still have no idea how to pronounce Lloyd. Two years later I'd worked my way through Lord of the Rings and had moved on to plundering the adult scifi/fantasy shelves of three different libraries.
Meanwhile, my brother and I had grown out of playing war outdoors or with army men indoors. Avalon Hill was pretty popular around our house. We had a subscription to the General for awhile and there was always something set up on a card table, half completed.
Ultimately, I started playing because of my parents. They'd picked up one of the earliest printings of the Basic Set, the one with the lizardman logo on the cover and the geomorphs inside. It was the winter of 1978 and I was 12, which makes it likely the guy had sold him back stock or maybe they'd picked it up early in the year. We always got at least one new game for Christmas, I think my parents were trying for something that I'd like as much as my brother liked war games. Most likely there were points were they regretted this chance purchase later. I have no idea what else I got that Christmas.
My older brother read through the rules and by the day after Christmas we were playing with a bunch of house guests, muddling our way through his quick map, using army men as figures. It kept us busy and the basics were mastered, we all controlled multiple men and rolled up more as they died. Much of the idea of the game was lost on us, like the Wisconsinites of old, we were mostly just playing a war game with magic. One of my friends stopped by at some point, enough to be impressed and want to play. My brother really only played a handful of times after that (years later), but never really got into the role playing or story part. The house guests went home, some got their own copies and kept playing.
I often think that was a pivotal moment. Holmes' earliest version made it apparent the whole game was up to you. There was no module to play or even that many rules. By the time my friend bought his copy after Christmas, B-1 was included, although as I remember it, it was as if someone had erased the monsters from it and you put them in yourself. Which was my first DM experience. By the summer my other friends had bought later copies and we tried out store bought splendor with B-2, but it didn't measure up to the dungeons we'd been making ourselves. The AD&D books trickled out, we added each one as we bought it. I bought Gamma World about that time as well and started buying the Dragon for pointers. Each new book meant we had to recalculate the rules again.
I think those early games, before Gygax's Advanced rule system came out set the tone for my friends and I. We were pretty much self taught and were often wrong, but we were junior high kids with an obsession. We devoted a lot of time to the game. We were tireless in converting our friends. We made elaborate dungeons. We played all night. We played for days in a row. We experimented. We made the world our own.