I would suggest right around the age of adolescence. SO, around 9 to 12. But, I think there's no one-size-fits-all. You can really choose when and where to talk to them about topics like that when you think they're mature enough or mentally prepared to address such topics. Developmentally speaking, though, I think 9 to 12 is appropriate.
I think ten years old is a good age. You have to remember that with sensitive topics like sex/drugs/alcohol/violence/etc. it's important that parents are the first people to bring it to their children's attention instead of them finding it on TV or from other ways.