The novel “Old Man’s War” by John Scalzi was published for the first time in 2005. It’s the first book in the “Old Man’s War” series.

John Perry just turned 75. He’s a widower so he decides to join the Colonial Defense Forces (CDF) that defend the Earth and other human colonies from hostile aliens. Together with other recruits who are the same age, he’s subjected to a transfer of his mind from his decaying body to a new one that has almost the same look he had as a young man but is much stronger and resistant and has much sharper senses.