Old Man’s War by John Scalzi
So when you turn 75, you join the Space Defense Forces. Kind of strange–the colonies only want people who’ve been around a bit. At least our intrepid hero is smart enough to realize that if it’s a two-year commitment that the SDF can decide to renew up to 10 years, that you’re really in for 10.
Old Man’s War takes us through a shooting gallery in space between humans and everyone else. We get to see the world through the eyes of John Perry, a 75 year-old green recruit who is getting his first taste of life off earth. Mostly, it wants to kill him.
It’s fun watching John grow throughout the book.