I'm not sure the Old Man's War universe really makes a lot of sense on extended scrutiny, but the justification here is a bit like mindstalk's idea about the cyborgs. To get into a milspec super-body, you basically have to die and have your mind uploaded. And it's a one-way trip from home, because the Colonial Union doesn't let the people of Earth even know anything about what's really going on off the planet. And, while admittedly nobody on Earth knows this for sure, you're deep in the shit forever after. So they recruit old people.

But there are also clone-soldiers with brand-new personalities (the Ghost Brigades of the second book), who seem to be used to commit war crimes for rather Enderlike reasons. So Scalzi's working both ends of the spectrum.