In the future, Earth and its colonies are defended from alien incursion by the CDF. The only way for humans to get into space is to be part of the CDF, which has fantastic technology far beyond anything any other human has invented. Inspired by an urge to see the universe and a distaste for growing old, any human is allowed to join the CDF on their 75th birthday.
The main character is John Perry, a white heterosexual male who lived all his life in a small town in Ohio. Somehow, despite the fact that the fact that Old Man's War is set in the far future, small-town Ohio is just the same as it was a few years ago. Same jobs, same modes of entertainment, same theories about sex and gender. Perry is a bland everyman who somehow keeps surviving and getting promoted up through the ranks of the CDF. The CDF itself is interesting, as are the alien races, and the battles between them aren't bad.