This is a serious talk. The world we are in now is a mysterious one, talking to children about sex; in my own understanding I will say it should be base on the level of understanding of that child. There are some children that are hyperactive, there are some that are very smart, intelligent and inquisitive, so we should take note of this kind of children so that we can know when to talk to them about sex. When you notice that the child is smart and has a level of understanding, we should talk to them about sex. At Least from age 3.