There’s some basic facts when it comes to comics– a.k.a. canon. A basic known piece of information about a character, event and/or anything in between. Canon combine with continuity makes a cohesive universe, and what comic book fans go off of. It’s in a way… commonsense.
There’s some across the board, well accepted and unchallengeable canon when it comes to the “main” universe of the DCU. A couple I can think of off the top of my head; Batman doesn’t kill (humans), Hal Jordan is most likely to punch his way out of a situation– I’m half-joking and half-serious there, but a main one (and especially so for this) is that Superman ages extremely slow in comparison to humans.
Now, I know, I know, what about how Superman aged thirty or so years while he was on Earth to what we know now? Well, Tom King (writer) posed that question via tweet,
If left alone, would Superman get old and then die from old age? On the one hand, no. On the other hand, he has aged (let's say) thirty-four years in thirty-four years; what's to stop the pattern?
— Tom King (@TomKingTK) January 10, 2018
I’ve always thought it was common sense. Getting old occurs when your cells reach the finite limit of capability to replicate. With Superman’s physiology, his cells process and thrive off of yellow solar radiation. Due to this (as far as we know) he may never reach that point. Thus, in theory, as long as he’s in the presence of yellow (or any that nourish his cells) solar radiation he can’t get old or die from “old age.”
Now, as to why Superman has aged “thirty-four years in the thirty-four years” part of King’s question: His body and cells took that long to adjust to the yellow solar energy, which as a Kryptonian, they weren’t accustom to. Think of it like someone going through puberty. The body changes, and forms to what is more “productive.” Now that he’s reached this, and unlike humans whose cells aren’t capable of what his are, his aging has stopped.
We’ve seen Scott Snyder (writer) touch on this when he teamed up with Jim Lee (artist) on SUPERMAN UNCHAINED. Wraith forced Superman to look at what future lied before him, and how his slow (if not lack there of) aging would affect him in relation to those around him,
Of course, this is all coming off of what looks to be King wanting a “corner office” in the name department of the comic book industry. King is challenging what has been, and I understand that’s a necessity (to a degree) when it comes to comics. Most recently, King’s done this with Batman and Catwoman, and the endgame of the two marrying.
His tweet seems to be foreshadowing a Superman story where he’d change a constant in the character’s makeup, perhaps to have Lois and Superman grow old together. It may not seem like a huge deal, but challenging what’s apart of a character’s makeup (in my opinion) skirts the fine line between what’s needed, and what a writer’s doing to make a name for themselves at the (possible) expense of the character.