When you're the minority in the universe, fighting for territory and survival, you need soldiers with plenty of life experience. That's why John Perry joined the Colonial Defense Force (CDF) on his 75th birthday. They promised him the potential for ten more years of life and a body as healthy and fit as he was when he was 20, but he could never return to Earth. Like many before him, Perry decided that this was better than creaking out the last years of his life, retired and widowed, in small-town Ohio. It wasn't until he began his training as a soldier that Perry started to comprehend the politics and the risk of his decision. Told in the first person, Old Man's War is a recounting of the first few years of his second life as a soldier in the CDF.
The first part of the story is propelled along by the mystery of why the CDF only takes old people, but once that is made clear, the action picks up and drives the plot through the end. Scalzi uses a combination of setting and dialogue to drop bits of background information and science to enlighten Perry and the reader, and at times they teeter on the edge of being info-dumps, but never completely fall over that cliff. And, nothing is ever fully spelled out, requiring both Perry and the reader to use their brains to fill in the blanks.
There are two things in the book that bumped against my assumptions. The first is the technologically advanced alien race known as the Consu. These are the first of many that Perry encounters in his battles across the known universe, and Scalzi spends enough time on the details of that battle that I was certain this would not be the last time they would appear in the book, and quite possibly they would be the catalyst to end the conflict over planets and existence. I was right about them appearing again in the book, but their intent remains a mystery to be unraveled later. Luckily, later already includes three more books (The Ghost Brigades, The Last Colony, and Zoe's Tale), so I will find out soon enough.