Not a geneticist, but I think even "cloned" tissue and organs risks rejection, as genetic degradation can be in even that first batch. I have an identical twin, and we would have, for the best part, identical DNA, except what might've changed/deviated over the years of our separate lives, and I'm no more willing to say I couldn't reject his kidney, or vice versa, than I might reject yours; the body is a weird unit, sometimes, and you'd still probably have to take rejection drugs, for the rest of your life. I seem to remember one of the MANY critiques of Erasure? (that Arnold Schwarzenegger film where he finds out they have copies of people, for this very reason); even clones don't work that way. Not wanting to sound like a ******, just trying to sound smart, about something I know rather precious little fact about.

Yeah, I assume that "rejuvenant treatment" can mean lots of various things, from giving you over to the Mechanicus, and having them swap out your internal organs, one or two at a time, and "pep drugs", that just help alleviate the signs of age (you're only as old as you feel?), all the way up to actual age-reversal serums, and weirder, that start to go of in the direction of "it works because the background says it is something these people can do." Like I exampled eons ago, Ascension has the Venerable Cal, and he has various life-support apparatus in his body, and more outside it, in the form of his chair. Certain augmetics, and definitely strange tubes, are rife on Imperial humans, and with no appearance of weakened status; hell, with the expense, and the AdMech's hold on keeping the good toys, one might argue that, to be seen with some cybernetics would actually be a status INCREASE, as in an Imperium known for seeing so many others as replaceable, you warranted the effort of preserving, so you must matter.

The first rule of 40k science is that it doesn't work like real science, but more like magic from a fantasy setting. 40k is, functionally, a fantasy setting with lasers, spacecraft, and aliens overlaid on top so it looks like sci-fi. You throw a little real science (and ideas from harder sci-fi) in there every so often to help maintain the pretense, sure, but for the most part, the question "how does this form of technology work in 40k?" is best answered with some variation of "a space-wizard did it".

So, yeah, clones may not work that way in reality... but 40k only bears a superficial resemblance to reality.

From Stars of Inequity :

Yep, one can't spend money on the other side. This also ignores the crazies who seek more "exotic" options, like Halo devices, but that's probably for the best; thos yahoos SHOULD be ignored, unless you are an Inquisitor, or work for one, in which case, please burn the heretic, if you can?

Life in 40K is cheap, and droves of people die every day ("drove" could be the new technical term for a grouping of your typical Guardsmen ). For those who DO survive, day after day, up into their later years, there could even be some justified self-conceit that they were alive because they were meant to be, and that "value" grants them the right, even the responsibility, to do whatever is necessary to remain healthy and vital, even as the grimdark continues, every next day, to make up for its slip up, and kill said person. Spend all the money, waste all the people, use any crazy gizmo? It's fine; the Emperor wants you to stay alive, and you just have to prove you're worthy by managing, whatever that entails. And I've rambled what was this for, so I'm going to disappear now. Have a good one.