When they start asking questions.
Me and my sister asked my mom about sex when we were fairly young, I was 7 and she was... 4? Yeah. I think it was 4.
Anyway, we asked questions about where babies came from, she told us, and then she went into extreme detail about the act of sex itself, about the clitoris, vaginal stimulation, oral sex, anal sex, vaginal sex, the effects of pornography, masturbation, the emotion that is involved with sex, how prostitutes should be legal, safe sex, how the man is bringing you down, the women being on top, sex toys, sexually transmitted diseases, condoms, etc....
We grew up a lot that day. But we at least we didn't grow up fucking stupid like this one kid I know. He's about 10, and his mom says she won't tell him anything about the subject until he's 14. I'm 14. That kid is going to be a bit fucked up in a year or two if he doesn't learn anything about the subject. Anyway, a good example of this, is he'll ask you if you would like to blow him. He thinks it means to go to the playground.
....