Trading that in for a decade of fresh life in a combat zone begins to look like a hell of a bargain. Especially because if you don't, in a decade you'll be eighty-five, and then the only difference between you and a raisin will be that while you're both wrinkled and without a prostate, the raisin never had a prostate to begin with.
So how does the CDF manage to reverse the flow of aging? No one down here knows. Earthside scientists can't explain how they do it, and can't replicate their successes, though it's not for the lack of trying. The CDF doesn't operate on-planet, so you can't ask a CDF veteran. However, the CDF only recruits on-planet, so the colonists don't know, either, even if you could ask them, which you can't. Whatever therapies the CDF performs are done off-world, in the CDF's own authority zones, away from the purview of global and national governments. So no help from Uncle Sam or anyone else.
Every once in a while, a legislature or president or dictator decides to ban CDF recruiting until it reveals its secrets. The CDF never argues; it packs up and goes. Then all the seventy-five-year-olds in that country take long international vacations from which they never return. The CDF offers no explanations, no rationales, no clues. If you want to find out how they make people young again, you have to sign up.
I signed.
"Paragraph five: I understand that by volunteering for the Colonial Defense Forces, I am terminating my citizenship in my national political entity, in this case the United States of America, and also the Residential Franchise that allows me to reside on the planet Earth. I understand that my citizenship will henceforth be transferred generally to the Colonial Union and specifically to the Colonial Defense Forces. I further recognize and understand that by terminating my local citizenship and planetary Residential Franchise, I am barred from subsequent return to Earth and, upon completion of my term of service within the Colonial Defense Forces, will be relocated to whatsoever colony I am allotted by the Colonial Union and/or the Colonial Defense Forces."
More simply put: You can't go home again. This is part and parcel of the Quarantine Laws, which were imposed by the Colonial Union and the CDF, officially at least, to protect Earth from any more xenobiological disasters like The Crimp. Folks on the Earth were all for it at the time. Funny how insular a planet will become when a third of its male population permanently loses its fertility within the space of a year. People here are less enthused about it now—they've gotten bored with Earth and want to see the rest of the universe, and they've forgotten all about childless Great Uncle Walt. But the CU and CDF are the only ones with spaceships that have the skip drives that make interstellar travel possible. So there it is.
(This makes the agreement to colonize where the CU tells you to colonize something of a moot point—since they're the only ones with the ships, you go where they take you anyway. It's not as if they're going to let you drive the starship.)
A side effect of the Quarantine Laws and the skip drive monopoly is to make communication between Earth and the colonies (and between the colonies themselves) all but impossible. The only way to get a timely response from a colony is to put a message onto a ship with a skip drive; the CDF will grudgingly carry messages and data for planetary governments this way, but anyone else is out of luck. You could put up a radio dish and wait for communication signals from the colonies to wash by, but Alpha, the closest colony to Earth, is eighty-three light-years away. This makes lively gossip between planets difficult.
I've never asked, but I would imagine that it is this paragraph that causes the most people to turn back. It's one thing to think you want to be young again; it's quite another thing to turn your back on everything you've ever known, everyone you've ever met or loved, and every experience you've ever had over the span of seven and a half decades. It's a hell of a thing to say good-bye to your whole life.
I signed.
"Paragraph six—final paragraph," the recruiter said. "I recognize and understand that as of seventy-two hours of the final signing of this document, or my transport off Earth by the Colonial Defense Forces, whichever comes first, I will be presumed as deceased for the purposes of law in all relevant political entities, in this case the State of Ohio and the United States of America. Any and all assets remaining to me will be dispensed with according to law. All legal obligations or responsibilities that by law terminate at death will be so terminated. All previous legal records, be they meritorious or detrimental, will be hereby stricken, and all debts discharged according to law. I recognize and understand that if I have not yet arranged for the distribution of my assets, that at my request the Colonial Defense Forces will provide me with legal and financial counsel to do so within seventy-two hours."
I signed. I now had seventy-two hours to live. So to speak.