My daughter’s almost four. We’re just teaching her “Mommy’s a woman, Daddy’s a man, you’re a little girl”. She confuses men and women in the grocery store all the time. If she said she was a little boy I would not react by cutting her hair and putting her on the football team; she hasn’t figured out pronouns yet, let alone stuff about girls and boys.
There’s no way a five year old “knows” he’s actually a girl.
When I was five, I was a space man...or woman. I dunno, sex really didn’t enter into it all that much. It was mainly about pushing buttons and blasting aliens. Just me and my dolly.