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.
As a musician, I was already very familiar with the British traditional ballads (the Folk Revival was underway and I discovered Francis James Child at about this time). Tolkien and books about him spurred my reading toward the older traditional literature of all kinds, both the sort that were the subjects of his scholarship (Beowulf and the Old & Middle English corpus) and its relatives like the Nibelungenlied, the northern sagas and the eddas, the Matter of Britain (King Arthur, the Grail), the Matter of France (Roland)) as well as the classics (Homer, et alia) and even, eventually, some of the Indian ancient poetry, the Rigveda, the Ramayana, and the Mahabharata (has anyone ever read the whole thing?).
I spent much of high school devouring everything I could find in this area, assisted by new releases in paperback of many of these works, as well as the scholarship that illuminated them, most especially on the topic of oral-formulaic poetry, where subject matter, linguistic form, performance requirements, and emotional power intersected so wonderfully. The traditional ballads (most of them) are the last hurrah of oral-formulaic poetry in northern Europe, and as a singer I could easily recognize the utility of the oral-formulaic process in performance, substituting equivalent phrases for ones imperfectly remembered in the heat of performance, or seeing fragmentary epithet phrases fossilized in absurd contexts (e.g., in the ballad/broadsheet of “Creeping Jane”, the racehorse lifts up her “lily-white hoof”, as any heroine would lift a “lily-white hand” — a convenient metrical phrase).