For me I guess I always had "it" in me, but I did not know exactly what "it" was; at least not till about 14 years ago and now I know I'm a Witch.
As a young child my grandma had this huge cast iron cauldron in her backyard. Whenever I was at her house, I was always in the yard playing with that cauldron and mixing "potions" in it; I have to say it always felt so natural being around that cauldron and mixing "potions" in it.
Not my Gran's cauldron, but I sure wish I had her's now
It was not until my late 20's and early 30's, after my divorce from my first (controlling ass) husband that I was free to explore who I was and the path I had always been drawn. That's when I knew I was a Witch, and when I finally connected with my spiritual side.
I would love to know when you knew that you were a Witch.
Many Witchy Blessings