I don't remember how old I was, but our son learned about "the act of mating" from watching Animal Planet when he was pretty young.
One day, when he was about 9 or so, we were driving down the highway with him in the back seat. Out of the blue, he asked how a human egg was fertilized - specifically, how the sperm and egg got together. I said, "You've seen animals mating on TV; it's similar with humans, but without the growling, clawing, and biting."
My wife's jaw dropped as she looked at me incredulously. Then I whispered, so only she could hear me, "Usually." ?