Old Man's War (1-4) by John Scalzi

Books 1-4

With his wife dead and buried, and life nearly over at 75, John Perry takes the only logical course of action left: he joins the army. Now better known as the Colonial Defense Force (CDF), Perry's service-of-choice has extended its reach into interstellar space to pave the way for human colonization of other planets while fending off marauding aliens. The CDF has a trick up its sleeve that makes enlistment especially enticing for seniors: the promise of restoring their youth. After bonding with a group of fellow recruits who dub their clique the Old Farts, Perry finds himself in a new body crafted from his original DNA and upgraded for battle, including a brain-implanted computer. But all too quickly the Old Farts are separated, and Perry must fight for his life on various alien-infested battlegrounds.
Have only described first book so I will not spoil plot