“For those who can’t appreciate a good story without having every little detail explained to them, and who can’t imagine any plausible reason for Children of Men‘s set-up (i.e., a world where humans can no longer reproduce), let me provide a possible reason: genetic warfare.
“At some point in history of this film’s world, someone engineered a virus that would cause sterility, and released it on a population in order to commit genocide. But it was an airborne and it got loose and became more virulent than predicted, and affected everyone that it came in contact with. Okay? Not that difficult to imagine, and good stories often let you use your imagination a little bit. Some people even enjoy using their imagination.
“But really the story doesn’t require you to know the why of the premise; the premise is plausible enough to hang the story on it, without having to explain why.” — Bkemper on Diggs.com.