I think parents should be free to teach their children whatever they think is best at whatever age they think is best.  There is never going to be a consensus about issues like this among everyone.  The problem comes when we have a one-size-fits-all public school system where regardless of when or if they decide to teach kids about sex, someone is not going to be happy about it.  If education was completely privatized, parents would have a choice to send their kid to whatever school most aligns with their desired curriculum.