Sorry, but that's a hopelessly broad generalization of kids' behavior. We have been traveling with our daughters since our oldest (now 15) was 6 months old. For whatever reason, they have always been very peaceful travelers. When they were younger, they usually just watched what was going on around them or fell asleep. Now that they're older, they just read. They don't scream, they don't kick seats, they don't do anything to annoy other passengers. In fact, we've gotten numerous comments from FAs or other pax about how well-behaved they were (which our kids don't get: "What did I do that was so great?"). One guy even told my younger daughter that it was his best flight in a long time when he sat next to her -- he had more than enough arm and shoulder room due to her diminutive size, with none of the annoying downsides.

And yet in your world, we would have been sentenced to sitting in the midst of a high concentration of potentially ill-behaved kids, simply because of their age.

The fact is this -- ill-behaved passengers are a pain in the butt to sit next to, regardless of age. Sure, I hate sitting next to a screaming kid, but I have been equally annoyed by many ill-behaved "adult" passengers as well.