A great many writers (perhaps most) have known they wanted to be writers all their lives, scribbling away in childhood, until finally some breakthrough brought writing to the forefront and they began completing and publishing their work.
But not all of us…
I have an intellectual background in mathematics, which (indirectly) led to a career first as a programmer and then as an IT executive in a number of startup software and computer consulting firms for almost 40 years. But, like many math-types, I also had a competing fascination with music, languages, and the visual arts. Everything, in fact, except writing.
As I’ve said elsewhere, it’s all Tolkien’s fault. I was a high-volume, indiscriminant, and rapacious reader as a child (still am), never going to grade school with fewer than half a dozen paperbacks to get me through classes, with a strong focus on science fiction and such fantasy as was available in the early 60s. My encounter with Tolkien when his first American editions and then the “authorized” editions came out in paperback, in early high school, gave me a sudden and immediate focus. In brief, I’m the sort of person who reread the Appendices obsessively, trying to understand why his hints at deep history worked so well, how he had built a world with so much consistent detail and background that resonated so effectively with his readers.