Honestly, at the early age kids are having sex nowadays, it is a wonder that it isn't taught to them the moment they come out of the birth canal.
That being said, I think that the earlier you start on teaching kids about sex, the better. By then they will just take in the information, and would not be as driven by their hormones as a regular teen or pre-teen would. You could always educate them on masturbation and alternatives to taking the edge off when they are in their early teens. I'd rather the both of us have to endure the most uncomfortable conversation ever, then bury them because they contracted HIV/AIDS, or being a grandmother before they are old enough to get working papers.