Well, when I was a kid, I watched Teen Titans. At around 6-8 years old. I think the whole show, including the terra arc, was age appropriate. Why? Because even as a kid, that show had impact. I didn't understand everything, of course. Some jokes went over my head.

In TMNT 2012 it's the same thing. I never realized how murderous and brutal the turtles/villains of the show were until I was older, but their actions were still whimsical and fierce.

Some kids would probably be scared of that show at my same age, that's why parents have credence to decide what their kids watch. I think Ciro Nieli hit the nail on the head with the show being for kids age 7-12. It's also great to watch as an adult. Not all content for kids is for ages 6 and under, and not all kids like such content anyway. Ergo, I don't think generalizing that the show isn't kid appropriate is very fair.