as soon as it becomes vaguely relevant (they start talking/asking about it, they see something on the news, their friends mention something, an adult in the vicinity says something relating to sex). if you "protect" children from knowing about sex, you are doing them a disservice. some people have no idea how much groping, sexual abuse, sexual assault kids endure when they're younger but the kid has no clue what's happening because no one taught them what constitutes appropriate touching vs. what is non-consensual & sexual touching.