I guess the education should begin at around 9 or 10. For girl child it should be easy but needs to start before they reach puberty and start menstruating. For both gender you could start the topic by discussing STDs, teenage pregnancy and when you see they are a bit comfortable talking about it, you can start using the names of sexual organs, the role of sex and in human society, their responsibilities, protected sex, age they should start exploring, and so on. Never, talk to both your boys and girls together. Girl to girl talk should be easier for mothers. And fathers should take care of the boys. Mothers may talk to boys but need to have a set agenda otherwise the awkwardness would embarrass both you and your child and never let you raise the topic again.