In 1962, writer and academic Umberto Eco published an essay called “The Myth of Superman,” in which he outlined how Superman (and superheroes in general) didn’t fit the traditional concept of a mythological hero due to the nature of capitalism and the episodic nature of Superman’s life. In essence, Superman has countless adventures over decades, all of which take place in a continuous present, while he remains the same approximate age. His story has a beginning, but it will never reach its end; but more importantly, he can never make progress, can never develop as a human being.Umberto Eco is essentially treating Superman as a mythical character, and utilizing this character as a sign for us as human beingEco was concerned with delineating the features of a 'closed' text - a classic Superman story is 'closed,' in Eco's terminology, because it is designed to elicit a predetermined response - the mythological iteration of the Superman character. Therefore, nothing can happen in a Superman tale which advances the hero along the life-path: he cannot marry, reproduce or grow old.
I think Eco's comment holds true for all comic book characters in a general sense. Their core personas have to remain the same so they are recognizable generation after generation. That said, what keeps comic books vital is each generation's different interpretations of core personas. The characters also appear in story arcs or graphic novels that take into account current events. Superman has definitely changed over the decades. In the Forties and Fifties, the stories were nearly always short and self-contained, but in the Sixties they began to take on a somewhat bigger scope. It was the reboot of the character in the Eighties that produced the biggest changes, though. Suddenly Clark Kent wasn't an orphan anymore, since Ma and Pa Kent were still alive. Lois eventually discovered that Clark was really Superman, and I think they're married in the current continuity, but don't hold me to that. The stories have become much more epic over the past 25 years, running for many, many issues and sometimes tying in to a multitude of other titles.
Whether or not this is a good thing is, of course, in the eye of the beholder.