When I was in kindergarten, my mother sat me down and told me about vaginas and penises and where babies come from. I was 5. She had her boyfriend draw me diagrams of a baby in the womb, and I had this book. As I grew up I knew I didn't have to be afraid to ask my mom anything. I feel like it was a healthy decision to tell me so young and I just wonder why other parents haven't done the same.
EDIT: Read the 3rd review on that page. What a gem.