Rejuvenat

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Rejuvenation (often shortened to Rejuve or Juvenant) refers to procedures in the Imperium of Man which extend the lifespan of a baseline human. The basic process involves reconstruction of the body with stem cells and tissue grafts, most of which are grown in growth vats. With rejuve treatments someone can easily live for centuries.

The Imperium being the Imperium, there's not enough Rejuve to go around. Mostly it's reserved for favoured servants of the Imperium (battle tested officers and commissars, higher ranking Administratum bureaucrats, Inquisitors and their retinues, etc) and for rich people who can afford the steep prices involved. If those on top can last centuries then they can better secure their position against upstarts.

Cell Draining

Now so far this does not sound so bad in of itself, especially by Imperial standards. Sure, it's exclusive and helps rich buggers stay on top of the dung heap but so were horses during the middle ages and people don't blame feudalism on Buttercup. Compared to things like gangs of deckhands towing shells into Macrocannons while being whipped or The Death Korps of Krieg it's positively benign. And when things are working as they should this is the case. That said, Rejuve Therapy does have it's Grimdark side.

There's always a high demand for Rejuve treatments and it's big money. As such, the demand for inputs is also high. This means that people which can supply the right tissue samples can make a lot of money and there are other ways of getting it than with expensive bioreactors. In short, a young person gets taken by some unscrupulous bastards which hook them up to a rig which drains them of their blood, bone marrow and stem cells. The bleeders are kept alive for weeks or months as they are drained until they give no more and their broken remains are discarded. In Varangantua the practice is to ensnare lower class teenagers at parties, but you know that kids would be targeted as well. Historically, the Imperium even used certain xenos as the supplies during the Great Crusade, but the demand for it was just as massive as it is now that those various xenos were drained to extinction just to feed the supply chain.

The scary thing about Cell Draining is that it's not done by Chaos Cultists, Xenos infiltrators, twisted Dark Eldar who glory in pain for pain's sake, the crazed fanatics like the Redemptionists or a brutal necessity of ten thousand years of warfare. There's a demand for a product, there's a quick and dirty way to get it and gangers (and occasionally other business ventures) are more than willing to supply it for cash by said means. If the Imperium was suddenly removed from all it's external threats, Cell Draining would continue on. Possibly it would expand as the market for Rejuve treatment grew. Thankfully in a rare bout of of common sense and basic decency in the Imperium, it's illegal and the Bastions come down hard on it when they get wind of it.

There are also alternative sources like Xenos. One, the Adarnians, were left alone during the Great Crusade due to being harmless but then were hunted to extinction when they were found to be a source of an elixir more potent than most other rejuvenats. Said poaching and usage of Adarnian rejuvenat elixir was illegal mind you, and Ezekiel Sedayne, Belisarius Cawl’s predecessor and a known user of the stuff, lamented being unable to synthesize their genetic code when they did go extinct.

Limitations

While the prospect of biological immortality without the usual complications sounds nice, the body gradually requires more treatments just to keep itself going, eventually coming to a point where the person in question simply cannot accept more treatments and has to come to terms with natural death. The Mechanicus and their allies have their own workarounds for this by simply replacing failing parts with mechanical equivalents, though anyone who'd want to go that route would have to butter up their local Magos, and knowing the Mechanicus they're not too keen on sharing their best toys even with that in mind. The exact limit tends to vary from author to author however, with five centuries being the closest to a widely-agreed value.

To give you an idea of the upper limits of Rejuve treatments, Tarasha Euten had arguably the best case scenario for a baseline human (being the foster mom of Roboute Guilliman on top of being the acting administrator of Ultramar while he was away meant it was easy for her to get on the priority list, and she lived during the Great Crusade and survived the Horus Heresy, two periods where the Imperium's technological base was much better than it is now), and even she succumbed to old age in the end. Still, she achieved something very few Imperial citizens can match: actually live long enough to die of natural causes.