Scalzi, John. Old Man's War
On John Perry’s 75th birthday, he did two things: he visited his wife’s grave, and he joined the army. The Colonial Defense Force, to be precise. When humanity reached the stars decades previously, they found that the universe is a very crowded place. Countless other intelligent species fight to colonize the same planets humans want, and some of those species have developed a taste for human flesh along the way. Thus, the Colonial Defense Forces were formed to protect those colonies humans have already secured and to toss the aliens off planets humans want to colonize. The CDF only takes fully mature adults, however, age 75 and up. Everyone assumes they have some secret rejuvenation technology to make the old young again, but no one knows what it is…no one but the CDF soldiers themselves, that is.
John quickly makes friends with a group of the other 75-year-old new recruits and they manage to stay in touch through training and beyond, from battle to battle with strange and diverse alien species. But when John encounters a Special Forces supersoldier who looks exactly like his long-dead wife but has none of her memories, he realizes that there is more to this endless war and to the CDF than he or anyone on Earth ever suspected.
Riffing on such sci-fi classics as Starship Troopers and Time Enough for Love, Scalzi’s Old Man’s War is nevertheless a fully-realized and unique view of humanity’s future among the stars, and was voted one of the Top Ten most influential science fiction books of the last decade by poll respondants on popular speculative fiction blog Tor.com.