Old man's war / Scalzi, John

To defend Earth from violent alien races, the Colonial Defense Forces recruits the world's elderly, hoping to create an intellectual army that will outwit the aliens and preserve life on Earth.

Enlisting in the army on his seventy-fifth birthday, John Perry joins an interstellar war between Earth and alien enemies who would stake claims on the few existing inhabitable planets, unaware that the conflict involves much more than he understands.