Oof, where to begin? A Monster Calls has death on every page, because Conor's mother's cancer is the central truth of the novel. No matter how much he tries to deny, avert, cheat, or even flat out ignore death, that ol' monster keeps right on knocking. In the end, though, Conor learns that while he can't avoid death and loss, he can learn to cope with them, which is the real truth he's been searching for.
To deny aging, like Conor's grandma does, is another way of denying mortality. She's just as bad as the rest of 'em.
When it comes right down to it, because he can't handle death, Conor's dad chooses his sick daughter over his grieving son, at the time his son needs him most.