Old Man's War - John Scalzi

Old Man's War is the first in a series of books about a future world where humans can enlist in the Colonial Defense Forces at the age of 75 years and receive a new lease on life. This story follows John Perry. John's a widower who's wife was planning to enlist too, but she didn't make it to 75. There are lots of aliens, battles, and some surprising twists. It's an old fashioned science fiction adventure, like the paperbacks I used to read when I should have been studying, several decades ago :) Highly recommended.