When John Perry turns 75, he does two things: He visits his wife's grave and he joins the Colonial Defense Force. The CDF's enlistment contract is incredibly tempting. When a person reaches retirement age, all they have to do is give up all their worldly possessions and promise never to return to Earth. In return, elderly recruits get to take advantage of the Colonial Union's secretive therapy, which somehow reverses aging. In essence, the soldiers exchange a few years of military service for a new life on one of the Union's many colony planets.
It would be easy to sum this book up as part Forever War, part Starship Trooper... and there certainly are elements of both - but there's far more. There's even an element that resembles the excellent way Karen Traviss has considered the meaning/signifigance of the clones in Star Wars. Brisk pace, and nicely presented characters give a great story and an exploration of the meaning of life and war - and the enduring power of love.