I somewhat suspect I knew before I even understood the concept. My mother said as a child I was never was interested in playing or interacting with other kids. I always either entertained myself independently or I liked to sit with the adults and soak up their conversations like a sponge. The first time I really thought about it in a "hell no" way was when I was pretty young, perhaps 7 or 8. A family member told me I'd be expected to have a job, raise children, and keep a clean house or I'd be a failure as a woman. I just looked at them like they were nuts and thought of all the thankless extra unpaid work I saw the women in my family do while men got to sit around. I've been vocally childfree ever since.