Old Man's War sits in the military science fiction genre but themes of the ethics of life extension, friendship, marriage, the significance of mortality, what makes one human, and individual identity are present within the novel. Aging plays its biggest role near the beginning of the novel with the CDF being able to find a way to reverse the effects of aging. The themes of marriage and friendship are explored in the characterization of John Perry through his continued love of his dead wife and his later meetings with Jane. When it comes to identity and humanity John Perry is the focus of attention and his characteristics by the end of the novel determine whether or not the reader believes John Perry is still human.