Old Man's War
by Scalzi, John
When humanity reaches the stars, it discovers that it must defend its claim to new planets against alien races. Alien cultures, morals, philosophies, and motives are unknowable. Only one thing is clear, livable real estate in the universe is scarce, and numerous alien races will have to fight for it. In order to protect the new colonies the Colonial Defense Forces recruit seniors on their 75th birthdays. Giving the seniors youth through an advanced life extension process provides the CDF with soldiers which have a lifetime of experience and wisdom.
Modeled on Starship Troopers, Old Man's War is about the meaning and importance of identity. What makes one human, the significance of mortality, and the ethics of life extension are all major themes.