They can have my crayons when they pry them from my cold, dead hand
Though I've talked about some of this before, I thought I'd repost my answer here:
I suppose you could say I got the crayons from my first grade teacher. I was in a combined first/second/third grade class at Buchanan Street Elementary School in Los Angeles when I was six. It was October and our teacher announced a Halloween short story contest for the class. All the entries would be read aloud, and the class would vote on the winner.
Most of the stories were happy little tales of ghosts and haunted houses. I, who liked to scare myself watching bits of "The Outer Limits" and "Night Gallery" on TV when I wasn't supposed to, wrote a little story called "Rattlesnaks [sic] and Cobras." It was a first-person story where the narrator gets attacked by shapechanging snakes in his backyard and dies.
A couple of girls cried when the teacher read my story. I won the contest hands-down. And that's when it crystallized for me. I was going to be a writer, no question.
I told my teacher and she starting letting me use the electric typewriter next to her desk. My first project at the typewriter was to try writing a "Star Trek" script. (Fortunately, that page is lost to time.)
I considered a lot of different ideas for day jobs during the next nine years, but writer was the one constant. Whatever job I ended up taking, I knew I'd be a writer too. I always wrote stories, often during class, but I would say it wasn't until the age of 15, in the spring of 1983, when I was a junior in high school, that I really got Serious.
My dad, trying to help me figure out what to do with my stories, found a copy of Asimov's on a 7-Eleven newsstand and brought it home for me. There were submission guidelines inside, so I sent them a story. A couple of months later I got my first rejection (a form letter with Shawna McCarthy's name at the bottom) and was crushed. But I kept at it.
My parents were encouraging, and couple of my other friends wrote the occasional story too, but really I was pretty much on my own. I subscribed to Asimov's, and during those first two years a couple of articles about Clarion appeared, one from a teacher's perspective by Algis Budrys, and another from a student's perspective by Lucius Shepard. In early 1985 I cajoled my dad into letting me apply to Clarion. He didn't want me to go, but he let me apply. I don't think he expected me to get in, but I did, and I went. I was 17.
Everything changed at Clarion. For the first time I wasn't the best writer I knew, but I was also part of a community of writers for the first time. I was pushed there in ways no parent, friend, or teacher had ever pushed. I received honest, if flipping harsh, assessments of my work. But maybe the most important thing of all was that all those older people took me seriously as a writer, and didn't condescend. There were no pats on the head, which of course made me feel more like a real writer.
Kate Wilhelm, during the last week of Clarion, called me the most improved student, which could have sounded like damning with faint praise from someone else but not from her. She and Damon told me to forget about setting any records, since I was already older than Chip Delany was when he made his first sale, and keep writing while I got more life experience. They predicted I'd start selling in about five years.
As it turned out, it took seven years* after Clarion and something like 200 rejection slips before I made my first pro sale. So I guess the watershed moments for me came at 6, 15, 17, and 24. Maybe I'll sell a novel before I'm 40!
* For two of those years, though, I was a missionary and writing almost nothing.