In English class, we regularly had to write stories and I always loved losing myself in the pictures I could create with my words. I loved writing almost as much as reading at that point. I'd read books constantly growing up. I loved Harry Potter but when the fifth book; Harry Potter and the Order of the Phoenix, my dad refused to buy more than two copies off of Amazon. He'd pre-ordered it for me and my brother but he'd told us that we had to let him read it too. So me and my brother sat on the stairs waiting for the delivery of our books with a rather anxious tapping of our feet and an impatience that was literally tangible. Finally the books arrived but after a couple of chapters my book was stolen away from me. It was my father's turn to read. I used the break in reading to watch them read jealously, constantly asking 'how much more?' Finally my brother finished his next two chapters and he gave me his book and so the process carried on. Looking back, it always makes me chuckle because I know that without my dad enforcing those rules, I probably wouldn't have stopped to eat, drink, sleep or pee. My bodily functions would have paled into insignificance compared to my book.
Nothings changed; I'm still an avid reader. J.K.Rowling inspired my first story. I'd always been fascinated with the magical world that she created in the Harry Potter books, but I struggled with all my unanswered questions. The biggest one of all was what were Harry's parents really like and so I started writing their story. I've never finished it - but everyone loves a bit of fan fiction - and it was a great place to start. It taught me several things. Firstly, if you have questions, you can always find the answer in your imagination. If you want to know someone's story, then write it. Secondly, every writer makes a world that you can dive into but only the best of authors make you want to never leave.
So, in answer of the question, there have been thousands of inspirational moments that have led to me taking up writing but I wouldn't say I ever just knew this was it for me. There are thousands of things I want to do and see in my life and writing is just a part of that but that doesn't make it any less important to me.