Have you ever seen a crude, Sharpie drawing of a schlong on a desk and thought, "wow, that's some grade A hilarity right there"? That's some classic 5th grade humor, which is something that nobody grows out of.
Nobody ages out of a certain level of joke-making that stems around the faux pas, things you can't say out loud, and wee-wee/doo-doo jokes. It's just funny. Maybe we're laughing because we're uncomfortable or maybe we're laughing because immaturity is sometimes very fun. Either way, if a 10 year old is laughing at something, I'm laughing at it too. Can't explain why.
Some silliness needs no explanation, it just is what it is. So if you've ever laughed at a fart in public, you might even fit into the immature humor category. It's not inherently clever or funny, but that's the point! It's silly, it's youthful, it's mindless and sometimes that's the kind of humor that can save a nation.