When people ask "How long have you been writing?" I'm stumped. There are too many answers.
It happened in stages, not all at once. I did not spring forth with "WRITER" stamped on my brow.
Some of the essential ingredients (in my case):
Imagination. There was never a time when I did not make up stories. It's how I got to sleep at night when my brain was too busy. I'd tell myself a story, act it out in my mind (sometimes playing all the parts) and somewhere I'd fall asleep. I still do that, though now it's usually part of something I'm writing.
Reading. I read voraciously from an early age, and learned most of what I know about grammar and sentence structure from reading fiction, from Jane Austen to Roger Zelazny. Lots of non-fiction, too. Curiosity is very helpful for someone who needs to make up whole worlds.
Decision. Dave mentioned this in his post on Monday. There is a moment of decision which for many writers happens when one reads a book and thinks, "I can write better than this." That happened for me. I don't remember the book I was reading (it wasn't memorable), but I do remember the moment and it was indeed a turning point for me.
Technical Tools. In high school I decided to write a Star Trek novel. It was Unspeakably Bad, but it was a gift to my future, because in the process I taught myself how to type. I improved on this skill in college when I got a part-time job typing technical stuff and thereby learned to use a word processor.
Other Writers. I moved in with a writer friend who needed a roommate. She talked me into joining her critique group. I wrote a story and showed it to them. It was Unspeakably Bad, but they were kind and encouraging to me, and I started writing more stuff. The writers in this group also introduced me to other professional writers in the area, great contacts for a fledgling career.
I've had good luck with critique groups, mostly. It's extremely helpful to have knowledgeable first readers, and it's also important stay connected with one's peers in the industry. They're the support group that every writer needs at the inevitable moment when the writer must be told, "No, you're not crazy."
Education. I learned about the publishing industry. The critique group pointed me in the right direction for submitting my work to professional markets. I read books and magazines about how to get published (not so much how to write). Later on I attended professional workshops, which also focused mainly on the industry and how it works. Invaluable.
Sales. After writing and mailing out a lot of short stories, some of which were Unspeakably Bad, I began to sell a few. One was bought by a magazine editor who later became my teacher and mentor in workshops for professional writers. Another was bought by an anthology editor who asked if I had anything else he could look at, and eventually bought a novel from me, my first book sale.
Mistakes. Oh, yes, the painful part. Everyone screws up some time or another, and I've made my share of boners. You learn from them, and go on. (Maybe that editor will someday forgive me for the late-night phone call...oops!)
The Most Crucial, Super-Secret Ingredient for Becoming a Successful Writer. Anticlimactic, I know, but in the final accounting the most essential part of writing is simply writing, as much and as often as possible. I've done that for as long as I've known how. Not always with the intention of becoming published—that came later on—but always telling stories. I do it every day. Here's a great post by Barbara Bretton about the importance of writing on a regular basis.
A writer is never finished practicing and honing her craft. There's always room for improvement, and always another story to tell.