When did you first realize you wanted to be a writer?

I began writing independently of schoolwork when I was nine.  On the one hand, I scribbled rhyming poems in pencil on the cardboard that came with my father’s laundered shirts.  On the other hand, I wrote essays on the structure and functions of the human body.  By the end of that year, I had drafted enough material for an illustrated manuscript on human anatomy and physiology.  This of course was never published, but it did anticipate my future career as a physician.