Ttyjukl

How long do Space Marines live on average?

What technology lets them live longer than we are living now or does it have more to do with their psychas?

A Space Marine enjoys the dubious type of longevity called "immortal until killed". Several Chapter Masters boast lifespans of over a thousand years of age. There are also Chaos Marines who are still alive (such as it is) for over ten thousand years, living in the Eye of Chaos.

• 
  Commander Dante is one of the most experienced and able Space Marine commanders. In no small part, this is due to the longevity of the Blood Angels, which he has ruled for 1,100 years. Dante is the oldest living Space Marine in the Imperium (excluding Dreadnoughts) and is held in awe by leaders of other Chapters, who can remember him being a famous commander when they were in the Scout Company.

• The average age of a Space Marine who is dutiful to the Emperor and repelling enemies of the Empire is about 300-350 years. No Space Marine has ever been reported of dying of old age and all of them expect to die in battle against enemies of the Empire.




• In the gaming literature, from which the characters are derived, most Space Marine Captains will have two or three of their hundred year studs on their foreheads as a sign of their superior resilience under battle.

• 
  Space Marines have been genetically-engineered to be paragons of destruction whose modifications and technologies take twenty years to complete and kill 80% of the people chosen to undertake it. The number of modifications and the nature of them ensure the only way for a Marine to die is on the field of battle. They are the shocktroops of the Empire whose lives are spent only on battles which affect the ultimate longevity of the Empire.

Since there are only a thousand chapters with an average of a thousand members each, spread across the millions of worlds of the Imperium, Space Marines are built to survive conditions no other fighting force in the galaxy can easily compete with; they have to survive fighting the most fearsome forces ever seen. Dying may be part of the job description, but not before a huge number of enemies precede them into death. Getting old isn't even calculated as part of their personal equation for life.

For eleven hundred years, I have fought and I have seen the darkness in our galaxy. I have seen the vileness of the alien and the heresy of the mutant. I have witnessed the sin of possession. I have seen all the evil that the galaxy harbours, and I have slain all whose presence defiles the Emperor. I have seen what you will see. I have fought what you must fight, and I have slain what you must slay... so fear not and be proud, for we are the sons of Sanguinius, the protectors of Mankind. Aye, we are indeed the Angels of Death. --Commander Dante - Codex: Blood Angels (4th Edition)

While Thaddeus' answer does answer the question as it was asked, I thought I would post this to provide some extra context to his answer by providing evidence of the longest a non-interred (as in non-Dreadnought) marine could live for.

In the novel Salamander by Nick Kyme, Brother Sergeant Dak'ir discovers the wreckage of a ship from the 154th Expeditionary [Fleet], part of the Great Crusade (Spoilers below - it's only background info for this answer)

Broken apart, forcibly disassembled, it was a ship nonetheless.



'One that crashed long ago,' said Illiad. 'Its reactor still functions
and we use its power to generate heat, purify the air and water. The
sodium light rigs are kept burning through the conversion of fusion
energy.'



'And this, a sparring hall?' Dak'ir had stepped out of the column to
approach the frame around the blast door. It had sunk into the rock;
or rather cave had grown around it. He tore at a section of it,
gauntleted fingers prising off a layer. Grit and dust came with it and
an origin stamp became visible beneath, fusion-pressed in blocky
Imperial script.


154th EXPEDITIONARY



Dak'ir shared a meaningful glance with Pyriel. The shattered remnants
in which the human colony had made its home had once been a vessel of
the Great Crusade fleet.


Page 272

On board Dak'ir finds (Book quotes in bold)

an ancient warrior, fused to the command throne of the ship, unable to move thanks to muscle atrophy.


"But the Fire-born sitting there was old, ancient in fact. His armour harked back to the halcyon era of the Great Crusade, when all Space Marines had been brothers in arms." Page 299



While the age of the Astartes in question is not directly stated however several passages do indicate that the ancient warrior fought in The Dropsite Massacre on Isstvan V e.g.


The venerable warrior bore the Legion marking of a trooper. His antiquated power armour was a deeper shade of green than that of Dak'ir's. It had a Mark V Heresy-pattern design with its studded pauldron and greaves. Page 299


Looking back at the old Salamander, Dak'ir was filled with a tremendous sense of sadness. He had watched his brothers stoically for millennia, keeping vigil until such a time as someone else took up his mantle or he could perform his duty no more. Page 301


'If his ship is indeed from Isstvan, he must be thousands of years old.' 'A fact we cannot be certain of,' Pyriel replied Page 301



Later on, on page 302, the ancient warrior does speak, adding more to the belief of the characters that he did indeed live though Isstvan.

Seeing as the book is set within the reign of Tu'shan as Chapter Master of the Salamanders and seeing as Tu'shan became Chapter Master 3 years before the Second War for Armageddon (which occurred in 941.M41) in the year 939.M41 and

considering that the ancient warrior could have fought on Isstvan V

a fortunate, or unfortunate depending on your point of view, Space Marine could live to be in excess of 10,000 years of age.

I know this is an old, answered question, but it's worth noting that in the novel "The Crimson King" it is actually revealed that Astartes aren't "functionally immortal"

In an age of peace, a legionary might endure for millennia or more, but he would not live forever.
The immortality of the Legions was a myth
Eventually the biological mechanisms sustaining them would fail and the horrifying descent into decrepitude would begin. Traditional life-prolonging drugs and surgery where ineffective on transhuman physiology. Martial cults spoke of a warrior's legacy as his immortality, but Hathor Maat wanted more.
Death held little terror for him, but the infirmities of age and the weakness of a failing body were a constant horror.

So in typical GW fashion, it appears there is new, contradictory evidence that would suggest space marines could in fact die of old age and are not immortal.

Ye Dante is old, so is Logan of the Space Wolves. He was alive and commanding Space Wolves during the first siege of Armageddon. ( 444.M41 ). Current date of the universe is in the mid 42 tho nobody agrees totaly on this matter.
By Black Library dates date is around 102.M42.

(Like a Phoenix on the Wing: The Early Campaigns and Glorious Victories of the Valhallan 597th, written by Lady General Jenit Sulla (retired) is published. Amberley Vail uses extracts from it multiple times within the Cain Archive.)

Try talking about Sigismund, he was a truly old warrior when he died, and it was even noted in his final fight against Abaddon that his speed had decreased significantly in his ancient body. He was still powerful, but this was also one of the most skilled, stubborn and downright legendary duelists of the imperium's history, so it is proven that an Astartes can age, the question is how powerful are the effects of time on the body. Because at his prime he would have been able to go toe to toe with Abaddon and win, after a thousand years of aging he was still good enough to cut through most traitor marines with ease. Not to mention how many years he lived during the great crusade. So while they can age, and they may be able to keep a form of consciousness for millennia (Not sure, the lore is confusing) they will degrade.