If they're seeing signs of maturity or puberty before that age. That's the sign that they're maturing too early. That can lead to their bones fusing too early so they don't get full growth. It can be caused by tumors and hormones producing growths in the body, so you wanna know about these clues. Listen, so many of the things we talk about, we wanna know if they're normal? We don't ask because we're embarrassed, but they're clues to us that things aren't going the right way. So from what I can tell, I don't think he's having premature puberty, so I think he is normal. It's okay to think those thoughts. You don't want him to be ashamed of what he's feeling.
I'll tell you as a doctor, but also as a father what I think is the right thing to do. I think if you're not embarrassed by the discussion and you answer it at their level, and by that I mean if they're asking, "how does sex happen" and you literally just describe it at a very common way not embarrassed way, then they just move on because they're truly curious about it. They're not lascivious in their desire to learn about that.
"The mom and the dad get together and then the baby comes out" and if they ask more detailed questions, take it as much into detail as they ask but if you're embarrassed, you'll make them embarrassed and it's also is a clue to them that something's going on and it's taboo which can attract their interest.