Lord_Enki said:
I think it's moreso the scenario of immortality, because showing the downside of one being immortal doesnt happen much. So artists like to portray it. *shrug*
I think it has to do with the fact that she still identifies herself as a human. She doesn't want to hang out with youkai, aliens, ghosts etc. because she doesn't WANT to. Also, it would be incredibly awkward trying to form actual relationships with beings that you've helped to hunt for centuries (if Perfect Memento is anything to go by).
Thus, her dilemma is not that of old age, but of knowing that the people she identifies with will never be as old as she is. And as old as some of the characters are, they're not immortal as a hourai is. The Earth would probably be destroyed and Mokou would just float through space until either being sucked-up by a black hole, incinerated by a star, or landing on a life-sustaining planet.