This is an interesting story especially for some of our older crowd. If you could be young again, but you would have to fight in wars for 10 years to complete the contract would you do it?
"John Perry, a 75-year-old retired advertising writer, joins the Colonial Defense Forces who protect human interplanetary colonists. Volunteers sign letters of intent and provide DNA samples at age 65, which John and his now deceased wife Kathy had done ten years prior to the beginning of the story. After visiting his wife's grave to say goodbye (as volunteers can never return to Earth), Perry takes a space elevator to the CDF ship Henry Hudson, where he meets Thomas, Jessie, Harry, Alan, Susan and Maggie, fellow male and female retiree volunteers. They dub themselves the "Old Farts".
Following a series of sometimes bizarre psychological and physical tests, Perry's mind is ultimately transferred to a new body based on his genetic material. His new body is a younger version of himself, but genetically engineered with enhanced musculature, green skin, and yellow cat-like eyes. He now possesses enormous strength and dexterity, nanobot-enhanced artificial blood, enhanced eyesight and other senses, and most critically, a BrainPal—a neural interface that, among other capabilities, allows Perry to communicate with other members of the CDF via thought."
By John Scalzi