Krypto has come back into Superman's life, but is a much older, less stable animal than he was in Superboys time. Earlier in the story Superman had decided to take the poor beast into space and maroon him on some alien planet in the interests of human safety(not exactly the sentimental type is he). Luckily another solution popped up with only a few pages to spare.
So basically Superman has a cure for old age, but rather than share it with the world he only uses it on his pet for fear of his own safety. He's got about as good a grasp on irony as Alanis Morrisette. Superman's not just a jerk, but a coward as well.
