My Mom read a book with me (specifically designed for parents having "the talk" with their kids) when I was 9 years old.
I remember stopping mid-sentence after reading "THE CRUCIAL MOMENT", crinkling up my nose, and saying to her, "I just don't see how the parts would fit!!!"
She smiled at me, nearly patting me on the head, and said, "Oh believe me, honey, they do!!!"
She and my Dad have been together since they were teenagers, and never had any other significant other but each other. I guess I could trust that she knew what she was talking about!