When to start sex ed?

While looking through the church bulletin this weekend I saw that they were offering "Wonderfully Made" classes geared to 5th and 6th graders. That made me think about my Ryan....he's 10 now and in the fourth grade, thinks girls are gross and kissing is disgusting. I started thinking about when I stop waiting for him to ask and start teaching him about his body and what's to come. Soo....I asked him a few questions just to probe around what he knows and doesn't know and came to the conclusion that he's very book smart but that's about it in this particular area. The questions and answers came somewhat like this...

Me: Ryan do you know what puberty is?
Ryan: Yes....it's the maturing of the sexual organs in boys and girls.

Me: Do you know what sexual organs are?
Ryan: Nope.

Me: Do you know what happens to a boy in puberty?
Ryan: His voice gets deeper.

Me: How are babies made?
Ryan: The mans sperm fertilizes the womans egg and the cells divide into an embryo and then the embryo continues to grow...........(etc...)

Me: Do you know how the sperm gets from the man to the woman?
Ryan: *shrugging shoulders and looking bored with the conversation* I dunno

Me: Do you know what sex is?
Ryan: Yes.

Me: What is it?
Ryan: It's another word for gender.

Me: Okay....thanks!
Ryan: Why are you asking me this stuff?
Me: Just wanted to know how much you knew.
Ryan: Can I play the wii now?

So now I'm off to buy/checkout the book Preparing for Adolescence by Dr. Dobson. Apparently he recommends teaching right before puberty- when the child will still think sex is "gross", but old enough to have interest and understand.

If anybody else has any ideas/thoughts/recommendations I'd love to hear it :)