Getting old sucks but as the old joke says, it's better than the alternative. However, what if there was a way to get to be young again? The catch is that if you do it, you'll probably die in some horribly bloody and spectacular fashion at the hands of aliens on a distant world. Any volunteers? 
 In this terrific novel, humanity has spread out to the stars only to find that they're competing with several types of aliens for habitable planets. The Colonial Defense Force has been waging those wars and gathering advanced technologies in the process. Needing volunteers, the CDF has a recruiting pitch that's hard to top. They'll take senior citizens from Earth and somehow make them young again. Survive the ten year enlistment, and the reward is getting set up on one of the colonized planets. 
 John Perry is a widower who joins the CDF on his seventy-fifth birthday. He quickly learns how the CDF makes good on their promise to make soldier young again, and also finds out that the war they'll be fighting is incredibly brutal and that most won't survive their enlistments. 
 The idea of humans fighting wars out in space with advanced technology is nothing new, but Scalzi did a great job of taking fresh approaches to this concept. The idea of an army made up of senior citizens is unique in itself, and the weapons and tech he came up with are also clever and inventive. It's filed with rip-roaring battle scenes, and the alien opponents he created are also several notches above what you usually get in these space war type books. 
 But the real hook is here the outstanding job that Scalzi did with the characters, and the sense of humor that he weaves into the story. There's a lot of very funny stuff in this book amidst the interstellar war, and that's something that many sci-fi writers forget to include. I particularly loved how Scalzi took the standard war story scenes of the gruff old drill sergeant training the recruits and gave it a twist. 
 I'll be checking out the sequels as soon as I can get my hands on them.