The Rite of Setesh - At about 500 years of age the strength of most human bodies is utterly spent and death becomes inevitable. However, some Imperial potentates refuse to accept their fate and instead they choose to invoke the Adeptus Mechanicus' Rite of Setesh. The subject’s body is mummified alive and permanently sealed within a coffin-like exoskeleton, where he or she can spend several Terran centuries in agony and claustrophobic horror before death finally claims the individual. Many members of the Mechanicum see the Rite of Setesh as a practice bordering on Tech-Heresy. Only the most radical Tech-priests would even consider performing the procedure on another person, and even then only in exchange for a fortune in money or political favours. The Mechanicum itself has no need for such cumbersome life-extending machinery. In their search for mechanical purity they have found better and more lasting ways to preserve their existence, by replacing the flesh entirely with machines rather then conserving it.

Psychic Powers
A few psykers and Sorcerers have mastered their art well enough to utilise the Warp's revitalising effect on their own bodies and thereby take control over the aging process. The greatest master of this ability is the Emperor, who kept himself alive for more than 40,000 Terran years before he was interred within the Golden Throne. He could even extend this power to preserve valued servants, such as Malcador the Sigillite, and this gift is still evident in many of the Living Saints of the Adepta Sororitas.

Halo Devices
Beyond those forbidden heights, gated and bound by bonds that man should never break, waits a throne that gods would fear to take.
Note found in the personal documents of renegade Inquisitor Erya Nephthys
The Hunger.png
The host of a Halo Device satisfying his unholy hunger for human flesh; the device has merged with his throat
On some Dead Worlds, in the volume of space at the rim of the galaxy known as the Halo Stars, travellers sometimes find alien artefacts left behind by long lost civilisations. The strangest and maybe the most powerful of these are the so-called Halo Devices. Many rich fools search for Halo Devices as an easy way to cheat death, but only the most unfortunate find what they are looking for. Prolonged contact between a human and a Halo Device does form a bond that rejuvenates the physical body of the human host, but at the same time it initiates a horrible transformation. The host stops aging and gains inhuman strength and resilience, but his body and soul begins to change. Visions of alien worlds fill his dreams and unholy thirsts plague his days.

After years of use the Halo Device becomes an integrated part of the host body, which itself mutates into an alien monstrosity. In the end nothing remains of the host’s human personality, but the cunning xenos menace which has risen in its stead is practically immortal and can even return from the dead to serve its unknown purpose.

ClatterSharp wrote:
Before The Age of Strife, Humanity had built the Men of Iron, develop space warp travel, and became so technologically developed that most aliens had to reluctantly sign peace treaties because Humanity was too strong of an opponent for them to attack at the time. So, from TAoS to the Great Crusade to the current timeline now, could Humans have lived about maybe 520-600 pre Eldar Empire Fall but now can only live up to 300-400? Sure they use eugenic treatments but maybe when they are in their late 200yrs of life?

What do you guys think.