Depending on what you are looking for.
The usual rating for Mature = Violence, Blood, Gore, etc.
However, some may view 'mature' as complexity, meaningful, sophisticated and so on. I like this definition more than the former.
In addition, many adults already have enough headaches in their lives, so another way to define 'mature' is simple, slice-of-life, and/or lighthearted. So in that way, a mature game is simply something they can use to relax. Any sort of provoking actions or excitements are subtle.
In anime/manga that are for young adult, it usually falls into the last one, sometime the second one. All the fighting, stuffs deem to be exciting (i.e. Naruto, Bleach--not really imo, but meh), usually fall to boy teenager category.
It can also be a mixture of the three.
There is also a post that Bod made somewhere about this topic as well; it is pretty accurate imo.
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