My nephew learned about sex in fourth grade, because my sister was pregnant and he was curious.
. I learned about sex in, what third grade? The basics. There is nothing wrong with teaching children where babies come from in fifth grade. Yes, maybe the stuff about safe sex and other such things should wait until 6th-8th grade and highschool, of course, but the basics are fine. Besides, I rather they learn about it in class, than on the playground.