Only One Thing Can Kill Superman - And It Is Not Kryptonite
Not only is he one of the faces of the brand, but Superman is among the most powerful DC heroes in existence. The Last Son of Krypton is a true powerhouse, with near-unmatched strength (unless put up against Marvel's Hulk, whom DC confirmed is stronger), speed, and durability, as well as a host of powers that make him quite a formidable combatant and capable protector of Earth. Sadly, all of this isn't to say that Superman can't be killed. As durable as he is, there's one thing that can definitively end the Man of Steel's life. And no, the thing in question is not Kryptonite.
Superman may be tough, but he's not immortal. He has an extended lifespan and his aging is slowed, but there will come a point where time itself causes his demise. His enhanced Kryptonian physiology will only get him so far before — like the humans he dedicates himself to — he will die of old age. Like most things in comics, however, Superman's natural longevity hasn't been set in stone. Based on specific factors, Kryptonians can naturally live anywhere from hundreds to thousands of years before dying.
Of course, it should be pointed out that Superman has died in the pages of DC Comics before, but calling even the most overt instance a true death wouldn't quite be accurate.
Even Superman's most definitive death wasn't definitive
As one of Earth's foremost defenders, it's no surprise that Superman has taken some heavy beatings throughout his comic tenure. In fact, several confrontations have seen him supposedly killed, be it through brute force or some form of tech created by one of his enemies. In time, it comes to pass that his death is reversible by some method; alternately, he never technically died in the first place, or the story was nothing more than a ruse. Even the most overt instance of Superman meeting his end, which could've rivaled all of the most tragic superhero deaths of the 21st century, turned out to be little more than a speedbump for him in the long run.
In 1993, DC Comics kickstarted the "Death of Superman" storyline, where the Man of Steel battles the ferocious alien Doomsday to the death. When the dust clears, he seems to have perished, with friends and allies promptly coming together to mourn the fallen hero. As the story develops, though, his death becomes far less definitive. Not only does the story leave some ambiguity regarding whether Superman actually fully died, but thanks to a Kryptonian regeneration matrix, he returns from the grave anyway. "The Death of Superman" remains an iconic DC story, but considering where it goes, the title is a bit misleading in hindsight.
Superman has fought aliens and robots, faced down cosmic cataclysms, and bravely faced enemies armed with Kryptonite, coming closer to death than any human could, only to come back swinging. Unfortunately for him, the only obstacle he can't overcome is time.