One of my first memories, when I was about four, was of my mother telling me no, I couldn't be called Mary, because I was a boy.  When I asked her how she knew I was a boy, she replied "because of your teeth".  She didn't tell me the truth until I was eight, when she realised that I thought that women had willies.  That was a real shock, as until then I was sure that she was mistaken about my gender.
(A few months later, in the spare room where I went to change the toys I was playing with, a Playboy magazine appeared.  For many years afterwards, I presumed that it had been left there by mistake.  I now think that it was my parents' idea of sex education.  I did find it fascinating but I didn't learn much about sex.  I've never really noticed women's breasts and Playboy didn't show full frontal in 1964.)