I don't have the greatest memory, but I remember that I didn't have much dysphoria until I was a bit older, around 12 or so when I got in Jr. High School. I think it's because when I was very young the few friends I had were mostly girls, and so we would spend our time playing "house" or "dress-up," and so I was perfectly content. I do remember wanting to be girl at this age, although it was never really at the forefront of my mind. I didn't really have any issues until puberty started to hit. That's when I realized that something wasn't right. I noticed that I didn't think or act like the other boys, and I would often fantasize about being protected or rescued (not that women need to be protected or rescued, it was just how I was projecting my feelings at the time). I tried to force these feelings down below the surface, partially because I was in denial, partially because my family has always been very conservative and religious, and I knew that they wouldn't accept anything less than a real "man's man." In retrospect, I realize that this was likely the source of the deep depression I fought through in my mid-teens to early twenties.
Still, it wasn't until I was about 18 that I finally realized that I
REALLY wanted to be a woman. It became a burning desire that would always resurface, no matter how hard I tried to suppress it. At first, it didn't affect my daily life too much. I could go days, or even weeks, without thinking about it if I carefully monitored my thoughts. But then I met another trans woman, and she told me about all of this stuff, and that's when I knew without a doubt that someday I would do something to change this body, to make it match what I feel on the inside.
Wow, long read. Sorry about the wall of text

Long story short: about 12, I suppose.