Under six. I don't ever remember a time where I didn't feel and think of myself as completely female, ever. I remember living in my home in Ohio and laying it bed and falling asleep thinking about being a girl, and we moved out of there when I was 5 (which is also around the start of my memories). It only intensified as the years went on, and I remember I developed coping mechanisms. I remember little girl me being 7 years old and getting in the car to go to church and thinking "I AM a girl, I'm just the only one in my family that knows it". That gave me a lot of comfort and that's just how I conducted myself from there on.
It's funny, because I never even vaguely felt this would go away, and when I was little, I *knew* I would HAVE to end up as a girl one day (even before I *heard* of transitioning). It has always been one of *the* most fundamental and integral aspects of my identity, and something that is constantly with me, every second of every day, and always has been.
Also, I decided I desperately wanted to transition at 16 but didn't have the guts to talk to my parents about it. I am sad I did not, and I remember the feeling that this was obtainable, but just not for me was the most crushing feeling imaginable. But I am happy to finally have started HRT 1 month after my 18th birthday, and have decided to not look back at time wasted