How young is "too young" for a girl to learn about periods/female anatomy/etc?

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I am not even really talking about teaching kids about sexual intercourse; I am curious if you think there is an age cut-off where girls should not be allowed to know the basics of human anatomy. I am from a rural area, and went to school with people who didn't know there were three different "holes" down there until we had "the talk" in class in 5th grade.