I think when they turn 4 no more underwear running around house. At what age about sex, now a days it is different, they are going to see this on tv, from friends, etc. You might want to start at 9 or 10. Before the bad cartoons, and sex on every channel I would of recommended 13, but if they find out the bad stuff about it before you get to tell them educated stuff. They probably will not listen.