I doubt many nine year olds know anything about sex. That is, most who don’t live in an urban ghetto.
I’m guessing those children who do, are exposed to porn, either from magazines in the home, or access to “adult” tv; they’ll put almost anything on tv nowadays. Over 40 years ago, my 8 year old cousin told my aunt that a FIVE year old girl in the neighborhood went around offering to perform oral sex to the boys in the neighborhood. They lived in a once good neighborhood that was in decline, especially after a housing project was built a few blocks away.
Our entire culture has changed in one generation- down the toilet!