I just saw another advertisement for an "Adult MMORPG" and had to stifle a snicker, because of the naughty connotation of "adult." But then I realized that many of our English words for maturity have come to connote sexually explicit content.

Is there a way that you could see referring to something as being for adults and/or people who are emotionally/physically mature without suggesting something prurient? If so, what is that way? Say, for instance, you want to use an adjective to indicate that a book was of interest to adults; you would say it is a "<adjective> book?"