Measuring mature of an anime is an odd way to judge anime, at least for me, because I tend to watch a wide range of genres and levels of seriousness. So when my friend, when talking in reguards to the new anime coming out this season, brought this up -- particularly the lack of "mature" subjects -- it got me thinking, what exactly makes an anime mature?
A lot of people would say things that contain "adult content", like violence or sex; however, having them doesn't really make the anime inherently mature, mainly because what is more important is how you present these types of content. Rather than just having them for the sake of getting that mature label, you treat each of the subjects like it is apart of the story and not something you inject into it; this basically treats subjects, like sex and violence, as if it is something special, and should be brought to the viewers attention at all cost. However, the viewer knows enough about these subjects to get the gist, and it should be a non-issue.
So if adult content isn't something that can be relied on, to make a story mature, what does? Well I've got a list that isn't necessarily fool proof, but it seem to work for me, and is basically the opposite of the shounen genre. Things like what I stated above is one, the others being: subtlety in it's message, nuance in conflict and characters, and an in-depth coverage of a certain topic. So lets go down the list and explain my reasoning more clearly.
Subtlety is Key
In a lot of shounens, you see the moral of the story showcased loudly and repeated often. This is an easy way to get young kids to understand what the point of its story is, which is fairly simple like: "Friendship is magic" or "Love concurs all". These are all well and good, but really the presentation feels like I'm forced to understand the stories intent, rather than interpret the meaning.
Just like adult content, the meaning of a story for mature anime should be woven into the story, to be figured out and understood as the plot develops. With this subtle nature, the overall intent can also be more up for debate, and the viewer ends up feeling more involved in the story, rather than just being told one.
Nuance Nonsense
It is very common to see stories with a definitive hero, and the anti-hero or villain. With these two characters, they usually have a clear goals that they try to follow, until the eventual clash.
Nothing is inherently wrong about these type of conditions, but it is an easy way to tell a story with a clear two-sided conflict -- which is basically the white vs black, hero vs villain, trope. Unfortunately in the real world, thing are more nuanced, and with nuance we are typically dealing with grey.
In a mature plot we aren't quite sure who is good, but both sides can be seen as somewhat evil. When these two opposing forces clash, you never really know who to root for, but you might have some bias toward one over the other.
With everything being inherently unclear, we sometimes have character that don't have a simple goal that can be followed easily, but they do have a goal and a vague idea of what to do. These types of characters are more human, and transcend the box most characters will be in, essentially becoming more believable.
Deeper Down
Last things on the list is basically a combination of everything working together, to make the experience less shallow and be more thought provoking overall. Both anime targeted at younger and older audiences can be pretty entertaining, but more often than not, things like shounen become extreme formulaic -- to the pattern of new enemy comes in, not strong enough, somehow beat the enemy because determined, rinse and repeat.
It is just easier to make stories like that, so you don't see a lot of anime pop up for a mature audience, simply because it is too hard to created. So the question is, what does make a mature anime? I'm not completely sure, but at the very least, it isn't simple and requires a bit more thought to be put together well.
I'll conclude with some anime that I would label as "mature", at least in my book: Shinsekai Yori (review), Koi Kaze (suggestion post), and Phantom (suggestion post). There are probably more out there, but these were the ones that I really like, and would recommend.