I'll disagree and say he IS too young.
My DD6 is aware that parts of the mommy and daddy mix together to produce a baby. And since her aunt had just delivered a little girl and she had expressed concern over her aunt's belly being torn, I did inform her that the baby comes out of the mom's pee-pee.
Further questions are typically deflected with, "You're not ready to learn about that yet, we'll talk about that when the time is right." Since we're typically very good about answering her questions, she accepts that.
