I don't think a mature anime needs to have any sexual connotation or violence, but it will have always something that is not proper to younger audiences. That is why I don't think there is any difference in labeling something as mature or not aimed for kids. One thing necessarily is the same as the other.
Using Luna's argument, for example, House of Five Leaves doesn't have any sex or explicit violence, but all the characters act against the law, even if sometimes for good reasons. How can an immature mind grasp that concept?
I think maybe I'm just coming at your argument with a certain age range in mind? Because I agree with that.
I think that maybe it's because of the shows I grew up on, I consider "kid" range to start around 8, at which point I feel there at least isn't any real language barriers for most shows/movies. My bread and butter growing up was dinosaur and astronomical documentaries and specials on the Discovery channel networks, so I learned a lot about physics and biology early in life.
For instance, I remember watching .hack//Sign when it first aired on I think Toonami, when I was 13 according to Wikipedia, and the main problem I had with it was that I kept forgetting what night it came on and would miss episodes entirely and be lost when I tuned in again. I mean, a lot of the psychological issues that it tackles went over my head, but I don't think it mans I shouldn't have watched it, and by extension I don't think that means children shouldn't be exposed to topics that they won't understand for years to come.
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@ Altimis:
I have a lot more ichor in my blood-circulating flesh-balloon reserved for Attack on Titan, but yeah, I am not glad that I watched SAO and I wish people could see why it's so bad.
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Most programs directed to children are morally wrong, if you look with great care. The main difference between those and the more mature ones are the focus of the story and the consequences of those acts.
As I've recently heard and am fond of repeating, "kids deserve our best, not our leftovers", which is why when I have control of the television I don't let my nieces watch anything that I wouldn't watch myself -- so, no Teen Titans Go, no Uncle Grandpa, no Spongebob Squarepants, etc. I'm trying as best I can to indoctrinate them with shows like MLP, SU, AT, and Avatar. Good animated movies do well, too, though. The next opportunity I have, I'm having them watch Spirited Away.