Old Man's War
The premise of the book is that humanity has achieved interstellar travel. And at the age of 75 every person on Earth can volunteer for duty in the colonial defense forces. Saying yes means that you will leave Earth never to return, you'll be serving 2-10 years as a soldier, and at the end of that time you can retire onto a colony. The assumption is that the colonial defense forces have some technology to make people younger otherwise a 75-year-old soldier wouldn't make very much sense. Saying anything more than that would be a bit of a spoiler for the book.
I enjoyed the book. It was a pretty easy read. The ending felt a little weak but I didn't mind because at least it had an ending.
I have read some of John Scalzi's other more recent books. It's clear that he hasn't quite settled into his story pattern when he wrote this book. It doesn't have that same twist at the end.