One of my students asked me this summer when I decided I wanted to be a writer. I had to think about that a bit and finally said, "I don't think I decided that I wanted to be a writer. I just was one."
For as far back as I can remember, I always saw a blank piece of paper as a challenge. It sits there, asking/ challenging / demanding that I put words on it. In a box I still have marble notebooks -- remember them? -- from elementary school that were never completely filled with classwork; when the school year ended, the pages with math exercises or spelling words were torn out, leaving me with any number of blank pages. The earliest ones include adventures of the Elmont Junior Detectives, my homegrown version of the Hardy Boys starring my friends and me.
Writing was combined with my feeble attempts at being a comics artist in more notebooks, as I composed single-panel gags starring Penelope Jones and his friends as well as comic strip-style adventures of Superduck (who was not a duck), Secret Agent George P. Wombat, and Johnny Applepix.