I believe that all children should be taught what sex is about, the dangers, the responsibility, etc as soon as they are old enough to understand what you are talking about.
The reason why this is such a contentious issue amongst so many parents is not so much the moral issue of whether the child should know these things at such a young age, but stems more from the fact that the parent themselves is too embarrassed to discuss the issue with the child… and so want to pass the responsibility to someone else (and so also blame them when things go wrong).
Of course every child should know all they can as early as they can, so that they learn the truth properly and know the dangers and responsibility that goes with it. The argument that teaching young children encourages them to have sex younger is ridiculous. The only way that this could happen is if the child is not taught about it properly, and so still feels the need to find it out for themselves. Much of underage sex comes from the fact that the child hasn’t has it explained properly and fully, and so feels the need to find the answers for themselves.