Why is it that here in America we have the idea that animated automatically equals childish? Animation started out as something very strange a quorky that was definitely meant for children, but when Disney, for example, released its earliest movies, they were actually marketed for adults. Snow White and the Seven Dwarfs was presented as a bringing to life of a classic fairytale that everyone had heard before, only now they could actually watch it happening. There are even scenes near the end where narration is told by words standing still on the screen. Young children would not have been able to read this, and may have had trouble sitting through the entire movie without getting bored.
As time went on, cartoons lost appeal in this country for most of the older generation, and so animators began to target a younger audience. Cartoons maintained their silly light-heartedness and lost their serious edge. However, this has not been the case in all countries of the world. In Japan, animation (which they shorten to "anime") is considered a legitimate method for story-telling. True, they have some animes that are meant specifically for children, but also many that are definitely not. I would give a good deal of anime a PG-13 rating for violence, language, or other content that could be inappropriate for children.
Ever wonder why anime has become so popular with the teen/young adult generation lately? It's because as anime and manga are making their way over to this country, young minds are discovering a hidden potential that the majority of Americans have passed over. Anime can be exciting, romantic, dramatic, realistic, fantastic, comedic, and anything else that any genre of movie can. While American adults who have grown up with the idea that animation is for children and hence it is stupid and not worth their time are less likely to discover what they are missing than younger generations who have not yet succumed to the stereotypes of society. Anime has its own style, to be sure, and that style is not necessarily similar to American animation. I don't see any reason why we should expect the Japanese to be American. It has become the norm in their industry to illustrate with things like "big eyes," dramatic hair, and chibies. Many Americans take one look at these things and dismiss anime altogether because this style is not what they are accustomed to. But the fact of the matter is that many have also been able to accept a style different from our own, and have discovered the amazing stories that anime and manga have to offer.

Seriously, does this look like a kids' story to you?
This is from the manga "Tsubasa Reservoir Chronicle" by Clamp. It's an intense and awesome story, but I would definitely say it's PG-13.
Now in all fairness, Disney (and especially Pixar) are finally starting to realize that cartoons can be meant for more than just kids. For one thing, it's parents who have to take their children to see the movies, and it would be nice if they could enjoy it too. With movies like Finding Nemo, The Incredibles, Wall-e, and others, Disney and Pixar have created masterpieces that are actually entertaining for all ages. There is a fun and simple level for the kids, but also a deeper and more plot-oriented story for the adults to follow. I have to wonder if this new development has anything to do with the opening up of the concept brought in by the anime genre in recent years. I can't say for sure why it is happening, but I can say that the change toward more rounded animated movies is quite welcome. Hopefully more Americans can come to realize the potential of anime and will not condemn it as a stupid past time for children before they give it a chance.
(By the way, the anime that are normally translated into English and releases on American TV are usually the kid ones, because--again--we expect animated things to be for children. Unfortunately, many Americans have let these sappy kid animes influence their view of the genre as a whole).