[BabyPiNK_FL]

Children are already asking where babies come from at that age and can usually understand simple and logic information so I don't see a reason not to begin introducing the biological information in addition to "stranger danger" good touches v. bad touches etc.
I think by 4th grade the sexual parts need to be introduced because when I was in 5th grade (in a good Baptist school) I knew at least one set of kids (who were very well developed) in 6th grade were having sex. I also remember discussing sex very frequently with my friends from at least 3rd grade. We were very good kids and never thought about doing those things, but we knew what they were and how it was done, mostly from movies and tv and R&B and rap song lyrics were very graphic as well. (Gotta love Miami Booty Bass!)