Part of my DC Earth-2
Clark Kent is a refugee from the planet Krypton. He came to Earth decades ago, has had a long superhero career as Superman, and lived out a long life with his wife Lois. But over time something became abundantly clear, Clark wasn't aging like a normal human. In fact he has far outlived the Clark Kent persona. Lois died of lung cancer almost 30 years ago at the age of 58, and yet Clark still looked like he was in his late 20s. Now Clark is a single father living in Smallville on his family farm with his new girlfriend Lisa Lasalle. And as Superman, he is still the world's greatest superhero.
Based rather heavily on the Kingdom Come version, with a bit of All-Star Superman, The Dark Knight Returns, and the Earth One comic thrown in. His origin is similar to the miniseries American Alien (if you've read that, it is a great retelling...but couldn't possibly work in the main universe). As for the slow aging thing, I took that from the series Invincible, but in this case prolonged exposure to Earth's yellow sun has increased his powers until eventually he simply stopped aging.