My five-year-old daughter knows that once a month, when the moon is half full, I have my period. She notices the moon as it changes, and often asks me on the days my period comes, or one or two days before or after, if I have started bleeding yet. She knows that the blood would stay inside me to help a baby grow if I were pregnant.
She doesn’t have a concrete understanding of sex yet, but she doesn’t need to. She is starting to understand my body, and by extension what her body will become. That’s what early sex education means.