Who you are, is who you've always been.

I didn't always want to be a writer, although I guess I have always been one and didn't know it.
If you read the countless books, magazines and blogs on the topic you will overwhelmingly find that writers are born, not made. You can't just decide one day that you are going to be a fiction writer and actually do it. You may have the technical skill, and you may have the necessary knowledge, but you may lack the most important element, the need. Writing is something you need to do. You can't quit coming up with stories to tell and you just have to get them out of your head.
Granted even if you have the need, you may lack the talent or you may never take the plunge and actually write something, but somewhere in the back of your mind, the need is there. Maybe you can suppress it and do something more successful with your life, but the desire will always been there in some capacity.
I honestly think that most people are what they will become long before they realize their calling. As a child I wanted to be an aerospace engineer, a superhero, a comic book artist, a psychiatrist, a teacher, a rock star, and a politician. Not once did I ever consider writing as a viable career choice. Granted it still isn't a viable career, but I'm trying to change that.
My mom was a writer and spent years chasing the elusive big book deal. Rejection letter after rejection letter she kept on writing. Why would anyone submit themselves to that kind of punishment? For the only reason there is, she needed to do it.
Whether you attribute it to growing up with a writer as a parent or too much time reading comics books and watching movies, I always had a vivid imagination. There was always a story developing in my head. Sometimes I found it hard to concentrate on such mundane tasks as school work or my job when I had a hero trapped in a volcano that was about to erupt. How could I leave the poor guy just sitting there waiting to die while I did a price check on detergent or took a math test?
The older I got the more detailed these mental movies became, the heroes developed lifelike personalities as did the villains and supporting characters. I not only knew them by name, but exactly how they would handle nearly any situation. Don't get me wrong, I knew they are imagined, but when you pour so much of your time, creativity and personality into them they take on a life of their own. Much like Gepetto's little wooden boy, they become real.
I was in my mid-twenties when I realized I had been a writer all along. Sure I still want to be a musician or a comic book artist, but those are just other ways to do what I think I was meant to do all along, tell stories. I'm a writer, I always have been and regardless if anything I write ever gets published, I always will be.