S.A.
I think that at ages 9 and 12, you definitely need to have the talk with both of your boys, especially the 12 year-old. Unfortunately, some 12 year-olds experiment with sex and oral sex. You want the information he gets to come from you, and not other kids at school or from movies etc....Get some books and give him a very direct talk about the anatomy and how it works. Make sure you make it clear that only consenting adults, or married people (whatever your beliefs are) should have sex. And make sure he knows about protection and the potential consequences of sex. Good luck!