I have to reply to this, because this reminded me of a story I heard about a week ago when when a friend of my mom's was over and I ended up joining the conversation.
There's this little kid named Andrew. Bout 3 years old, and she babysits him every weekend. He sleeps in a bed in the living room, and the parents bedroom isn't that far away from there. One night, around midnight or so they say, he woke up and knocked on the door and said "Are ya'll awake?" She said "Yeah, go to sleep." There was a pause, and then the kid said "Are ya'll having sex?"
True story. The kid knows what sex is at the age of three. THREE. Apparently, his parents have let him see way too much...