Reply #16: 5th grade, I think.

[The Velveteen Ocelot]

They sent the boys into another room and showed us a rather cryptic movie about what Kotex is for. I remained rather puzzled about the whole thing, and the movie did not enlighten us in any specific way about why, exactly, one would ever need Kotex. This was in 1961. We eventually learned the details about where the various body parts go from the older kids.