I think you will find, if you examine his response below, that he absolutely does reject the idea of playing with kids. He thinks D&D isn't for kids and is only for adults.
I don't have to "think about it" I've not only played with kids but I was a kid when I learned to play. As was the vast majority of the people on this board and the players in the world. It says right there on the cover of the game that it's for Ages 12+ (which means "younger than teenager"). And MANY people have found it's fine for ages younger than that.
I am not harshing you for it being not your personal preference to play with kids. That, to me, makes sense. And I want to be clear about that. If it's not for you, it's not for you, and I am not telling you that you should play with kids or should feel any obligation to do so.
I am harshing you for suggesting that the very idea of kids playing D&D is objectionable. That you think no kids can be mature enough to play it. That you think kids would by definition break the game. It's an absurd claim that deserves some mocking, frankly. You are SURROUNDED by people who learned this game young, and did just fine with it. The entire game was popularized by kids playing it in the late 70s and early to mid 80s to begin with. There is a reason it was in E.T.. There was a reason it was in Stranger Things. There is a reason it was a kids cartoon. It was MOSTLY a kids game to begin with. And while the game has changed over the years (though I would argue it has not gotten more complex), kids STILL play this game. It's still intended for audiences which includes pre-teens in addition to teens and adults.
And frankly, D&D doesn't need to be mature or serious. It can be crazy and zanny and be a fun game. So even if you think kids can't handle the oh-so-serious version you seem to enjoy (though many can), others might enjoy the crazy and zanny stereotype you think kids would bring. So even if your claim were true (and in my experience it is not) it still doesn't make sense to make that generalization and conclude they shouldn't play.
So while I respect your right and preference to not play with kids, I darn well don't agree with your claim that "I just think D&D is not for kids". Oh it's for kids all right. If not for those kids in an earlier era popularizing the game, you might not be playing it today.