We're homeschooling Patrick this year, like we did for the girls during 6th grade. He's 11. We were just riding in the car when he asked "The Question".
I guess it's the same age I started getting curious about such things, too. They say you shouldn't overload them or go way beyond what they're able to understand, but you don't want to obfuscate or confuse, either. Incidentally, if the girls have asked such questions, they went to AnnaMarie. Quite frankly though, I know a lot more about it than she does. It's one of those things she'd just as soon not even think about. It's there, it's a necessary part of life, that's it.
So you prompt them to ask specific questions. That way they are steering the conversation to their level of understanding. "How does it all work?" "How do I get things started?" "How do you know what to put where, and when?" "Why does Mommy's look different than Daddy's?" All good questions.
Since we're homeschooling I asked the charter lead from the district. She told me about visible models that the district would even pay for. Patrick and I could do the assembly together and we could discuss everything's names and functions as we go along.
We're really enjoying the time together.