There were some inconsistencies going on pretty much ever since I was born, which my mom is now telling me that she always noticed, namely my more feminine social behavior. But since the question is not about when these "symptoms" actually started showing up, but rather when I noticed them myself, that answer would be at about age 13. As a kid, I didn't really care. I was myself, and I didn't really have much of a concept of gender beyond the traditional stereotypes. And since I didn't have many stereotypically feminine interests, I didn't really have any reason to question.
Middle school was when things RAPIDLY changed. While boys and girls are pretty much physically the same as kids, they start rapidly diverging at puberty. And that was when I started feeling like what I had was wrong, and that I should be a girl instead. Thinking about my voice changing, and having a male sex drive, and getting more buff, and being forced to wear loose baggy shapeless male clothing all really started getting to me. (Unfortunately, though, it wasn't until age 27 that I discovered that I could actually do something about it.)