Old man's war
(Book)

John Perry did tow things on his seventy-fifth birthday. First he visited his wife's grave. Than he joined the army. It's a rough universe out there, and the bulk of humanity's resources are in the hands of the Colonial Defense Force, which shields everyday Earth people from too much knowledge. What everybody knows is that when you reach retirement age, you can join the CDF. They don't want young people; they want people who carry the knowledge and skills of decades of living. You'll be taken off Earth and never allowed to return. You'll serve two years at the front, fighting ever-changing alliances of hostile aliens. and if you survivie, you'll be given a generous homestead stake of your own, on one of our hard-won colony planets. John Perry is taking that deal. He has only the vaguest idea what to expect. Because the actual fight, light-years from home, is far, far harder than he can imagine--and what he will become is far stranger.

860L,Lexile