The Monk just finished Old Man's War, a scifi adventure story that is essentially the grandson of Robert Heinlein's excellent Starship Troopers (the book, not the horrendous movie). In OMW, septugenarians are infantry recruits for the Colonial Defense Forces that protect humanity as it expands through the stars.
Why use a bunch of decrepit oldies? Life experience. The technology exists to reconfigure the recruits' physical capabilities (saying anything more would give away too much) but the CDF wants the ethics, knowledge and life experience of the grandparents who can understand the concept of fighting for abstract "humanity" more than an 18-year old kid suffused with his or her own narcissism.
Essentially, the book asks what would you do if, at age 75, you could be physically improved to youth in exchange for a two-year enlistment in the army, with the army's option to retain you for up to 10 years total, a 70% death rate in those 10 years, but the ability to retire to any human colony (no return to Earth) and live out yet another adulthood of 50+ years?