Same goes for sex (not teaching kids how to sex, mind you). Studies show time and time again than introduce children to the concept of sex when they're younger leads to safer sex practices, more sexual confidence, lower rates of teen pregnancy, and lower rates of STDs when they grow into adolescence and adulthood.
They're going to do it, like it or not, and keeping their "poor innocence" in check isn't helping them. You have to realize that innocence is really just a projection you're throwing onto them. They aren't any less childlike if they know how babies are made, or if they learn what their genitals can do aside from making yellow streams. Children are very naturally sexually curious by age 5 or so anyway, they just lack the understanding of what they're curious about because parents don't teach them.
Then parents get all shocked then they find little Billy and Suzy playing "doctor" and instead of properly explaining why they shouldn't engage is such play, they create a negative stigma towards sexual conduct all together that can leave lasting affects well into adult life.
I've had too many partners that are completely uneducated when it comes to sex, and have psychological anxieties that affect their performance and such.