I have no personal experience of it, but I know people who have. As a result I've read a bit about it. It seems that people in these circumstances can be content with who they are, but the external pressures from society can be overwhelmingly unbearable.

I don't know how common it is, but you can look up the terms "non-binary" and "transgender" and find out more.

Again, personally, I think we should just let people get on with being who they are. Respect them for who they are and the work they do, and just stay the hell out of their personal lives. No business of mine.