Here's a fascinating story about a six-year-old in a male body knowing that she is really a girl. And she is one of triplets. Interesting how these things happen in nature.
The thrust of the story is that the school won't let her use the girl's bathroom. But what I find interesting is that at six, she knows she is in the wrong body. In the last line of the story, she says they're being mean to her because they say she's a boy and she is a girl. She knows that at such a young age. Here's the link: