Well, it depends what we're talking about...
As for writing, I always partly lived in an imaginary world, making alternative version of the stories or making my own. Actually, it never occurred to me to write them down. It was sometime in high school that, during a boring class, with my friend we started writing a scene; made it more and more difficult for the character to get out of the situation. The story was never continued, although I still remember most of the plot I made later... I think this was when I realized that I can build longer stories and put them on paper.
My drawing was kind of second-effect there. I've been doodling a bit at the moment and thought that if I wanted people to see the stories as I do I might have to draw them in comic form instead of writing. I've been reading comics since I was six so it seemed natural to do that. But the moment I decided to actually learn drawing was when I saw a poster for drawing classes. I never went to those classes, but the poster had a realistic eye drawn on it; I looked at the eye and decided I am going to learn to draw so I could draw something like that. (even though I still have a lot to learn I know i already reached over the level of that eye some time ago :D It seemed amazing at that time, now I know it wasn't such a high-level drawing)
Becoming a writer and/or comic artist was the first thing I really wanted to become in life (apart from the fact that when I was little I wanted to be a singer - not a star, but a singer - but at some point the dream fell apart) and later I learned that writing and drawing/painting are the only things I can really do.