The intended audience for most "coming of age" stories is, first, teenagers, and, second, almost everyone else. I know that seems like a very broad answer, but consider the reality that "coming of age" describes.
"Coming of age" stories (like John Updike's "A & P," for example) illustrate a fundamental change in a young person's view of the world and his or her role in the world. In other words, a young person's understanding of the complexities of life usually grows as a result of a particular event or series of events, and he or she is never quite the same. Sometimes, the coming-of-age event is dramatically negative for the person involved, but most of the time, the experience--which might be painful--is a passage from the innocense of childhood to a more sophisticated and discerning view of the world.
Even though the primary audience is most likely to be teenagers--mainly because teenagers are the ones who will have a "coming of age" experience--the wider audience is anyone who has had such an experience, and that inlcudes most adults. Most adults who look back on their youth with some reflection realize that they, too, went through some kind of coming-of-age experience, and reading about that experience brings back what is perhaps a bitter-sweet memory of their own growth from innocense to experience.
So, stories about coming of age resonate with both young and old--the young are comforted by knowing that they are not the only ones who have undergone an experience that changed their view of life profoundly, and the adults are reminded that they too went through an experience that most likely enhanced their ability to make their way in this world.