My therapist reminded me recently that kids under eight or so actually think about if someone is a boy or girl and there are about 100 facets that can point one way or the other. Hit 70 and you'll always be identified as a female. Hit fewer and not as much. But adults don't think, they just go off one or two or three cues before deciding, like hair, dress, facial features, breasts, posture, etc. I want to learn more about these hundred cues.
Most times I find the kid isn't sure and defaults to identifying people as male, which kinda makes sense in our patriarchal society.