Would calling it "Sophomoric Adult" be a better description? We have to be careful not to conflate the literal meaning of mature with the meaning of mature as it is used for ESRB ratings. In the literal meaning, we are talking about a person who has a high sense of responsibility, self-accountability, and integrity. We are also talking about a person who is capable of putting the needs and desires of others before his/her own(the property of self-sacrifice). The mature person is able to take a balanced, reasoned approach to life and has a high level of self-discipline and self-control.
Mature in the sense that the ESRB uses it for ratings simply means that the material contained within is not appropriate for young, impressionable minds. The young, impressionable mind is not always capable of separating fantasy from reality, nor is it always capable of understanding the full consequences of its actions such to be able to engage self-restraint. It may end up imitating actions or utterances that it witnessed at inappropriate times or without fully understanding the consequences. Other times, it may not understand that a particular action is impossible to perform or that a particular state is impossible to obtain, and even when it comes to accept the impossibility of the things, it may not fully understand why. It just lacks the knowledge, insight, and complexity of thought to do so. Such minds do not always understand why certain behaviors or inappropriate or hurtful to others or are apathetic to the harm they may cause as a direct result of their actions. This is the sense of the ESRB mature rating, that the material is inappropriate for young minds that may not have developed sufficiently to exercise the self-control and understanding necessary to handle such material responsibly.
It can be argued that some adults are given to being impressionable and have difficulty with self-control and understanding the full consequences of their actions. Well, such adults are immature and are usually called such. It is true that age does not guarantee wisdom, but the inexperience of youth carries a high probability toward folly(essentially, young and foolish is a redundant statement).
Now, as for true maturity in games, I definitely agree that games, as a whole, are severely lacking maturity in the sense as I outlined in the first paragraph. Many games are really just sophomoric. They are like watching 13-year-olds playing at adult dress-up. The only thing that has really matured about video games is the graphics have gotten better and the play-time is extended(sometimes unnecessarily and beyond what is called for), but the actual content still takes the same immature approach to everything as they always have, be it love, sex, religion, politics, history, education, violence, or just the human experience in general. There is a complete failure to be informative, educational, or enlightening on such subjects, and the usual approach is to sardonically mock and sneer at those things or succumb to mere hedonistic revealry. The general attitude is often "give me what I want, let me do what I want, and then just shut-up and leave me alone", which is more the attitude of a 5-year-old.
Personally, I attribute the failure of video games to mature on the attitude that games must necessarily always be about having fun. While there is nothing wrong with having fun, it is not the only mode and means of enjoyment and enrichment of ones life. This failure to expand to other modes has stunted the cultural and intellectual development of video games for decades and is one of the prime barriers that I perceive as preventing video games from truly aspiring as a medium for expression. It is not necessarily bad having some things that tap the most base thoughts and instincts or are just simply fun. These can often provide quite a carathsis. However, these are not the only possibilities. Yet, the video games industry and community have pigeon-holed video games as a medium into such a narrow regime. Consequently, video games, as a whole and as an industry, have never done any more maturation than improve on graphical detail and extend play-time. But, based on the buying habits of us gamers, this is precisely how we like it to be; so, we should just stop pretending to anything more.
Addendum: An additional thought that just occurred to me, taking the approach to make a video game with a gruff, grim story-line is also not necessarily the mature approach; it only gives the illusion of maturity. While life as an adult can certainly be filled with difficult situations requiring difficult decisions and hardship, it can also be filled with much joy and fulfillment as well. It's often a matter of the choices we make and the attitudes we have as we go forward in life. It is the approach to the subject matter, not the subject matter itself, that truly delineates a mature(mature in the sense as I described in my first paragraph, not the mature rating) game from one that is merely pretending at maturity.
