You're 65 years old. And on your birthday, you get a letter from the army or rather from the Colonial Defence Forces. It goes something like this:
The CDF needs you! Join when you're 75 and we will make you young again. You will get to go to space, protect human colonists, meet interesting alien races and kill them. Unless they kill you first (which they probably will).
But you've stopped reading after the 'make you young'-part, didn't you? Of course you did. And so you sign up and when you turn 75, you join the CDF and they really make you young again, but maybe not quite as you expected.
Old Man's War by John Scalzi follows John Perry, who joins the CDF on his 75th birthday, leaving Earth for good and going through the process of becoming young again and becoming an elite soldier at the same time.
The battles he fights in are a hell of a ride and thrilling to read. I enjoyed the banter between him and the rest of the Old Farts (as they call themselves), a group that forms itself during basic training. I laughed out loud a couple of times when I read the book duringh my morning commute.
I also liked the alien races, especially the Consu who seem to treat war as a kind of hobby (although Perry quickly finds out that there's more to it than that). The book is something of a sibling to Heinlein's Starship Troopers and touches on political and social themes in the same way, making it much more than just a exciting science ficton story. Although, again like Starship Troopers, it can be read purely as that. But then you're missing most of the fun.
Like Pirate Cinema, I got the book from the Humble Bundle e-book bundle. I'm about halfway through all the book included in the bundle and so far there was not a single disappointment and a lot of great reading.