John Scalzi - Vének war (2019)

In The War of the Elders, John Scalzi describes a world where humans already live on space colonies and wage brutal battles for new ones under the leadership of the Colonial Defense Force. This is absolutely necessary, because resources are scarce, and there are few habitable planets, and because of this, emigration from Earth is severely restricted. For many, the only option left is to register as a soldier upon reaching the age of seventy-five, and after two years of front-line service, settle in a colony, being handsomely rewarded. This is not the only reason why this possibility sounds tempting: according to public opinion, the Colonial Defense Force somehow rejuvenates people, since you cannot go to war with old people.

So John Perry did two things on his seventy-fifth birthday. He visited his wife's grave and then joined the army. You only have two years to survive. On the other hand, even he has no idea what trials await him - the brutal wars and the light-year distance from home change a person forever. Something much more strange and dangerous.