Grey Templar wrote:

Ok, can you quote the relevant text? Because I suspect there were extenuating circumstances.

"‘Don’t talk to me about failure and lack of courage, Dantioch. You excel in both,’ Krendl spat. He bobbed his head at Serapis, the splinters of frag still embedded in his face-flesh glinting in the chamber light. ‘That is how the great Barabas Dantioch came to be left guarding such a worthless deadrock. Lord Perturabo’s favourite here came to lose Krak Fiorina, Stratopolae and the fortress world of Gholghis to the Vulpa Straits hrud migration.’
As Krendl growled his narrative, Dantioch remembered the last, dark days on Gholghis. The hrud xenos filth. The infestation of the unseen. The waiting and the dying, as Dantioch’s garrison turned to dust and bones, their armour rusting, bolters jamming and fortress crumbling about them. Only then, after the intense entropic field created by the migratory hrud swarms had aged stone and flesh to ruin, did the rachidian beasts creep out of every nook and crevice to attack, stabbing and slicing with their venomous claws.
Most of all, Dantioch remembered waiting for the Stormbird to lift the survivors out of the remains of Gholghis: Sergeant Zolan, Vastopol the warrior-poet and Techmarine Tavarre. Zolan’s hearts stopped beating aboard the Stormbird, minutes after extraction. Tavarre died of old age in the cruiser infirmary, just before reaching Lesser Damantyne. Vastopol and the Warsmith had considered themselves comparatively fortunate but both had been left crippled with their aged, superhuman bodies.
‘He then thought it wise,’ Krendl continued with acidic disdain, ‘to question his primarch’s prosecution of the hrud extermination campaign. No doubt as a way to excuse his loss of half a Grand Company, rather than laying the blame where it really belonged: the Emperor’s bungled attempt at galactic conquest and his own failed part in that. The IV Legion spread out across the stars. A myriad of tiny garrisons holding a tattered Compliance together in the wake of a blind Crusade. Our once proud Iron Warriors, reduced to planetary turnkeys.’
‘The primarch was wrong,’ Dantioch said, shaking his iron mask. ‘The extermination campaign prompted the migration rather than ending it. Perturabo claims the hrud cleansed from the galaxy but, if that is the case, what is quietly wiping out Compliance worlds on the Koranado Drift?’Pg.493 Age of Darkness"

 Grey Templar wrote:
Again, no. That is not what that means.

Hrud entropic fields mimic the effects of old age, but its not actually old age. He died as a result of being under the effect of the Entropic Field, which mimics old age in that you die due to organ failure. But an Astartes's organs won't do that naturally.

 VictorVonTzeentch wrote:
Unnatural aging at the hands of a weapon is not the same as dying of natural old age.

Warhammer 40,000, 6e, p.186 wrote:Although long-lived, the primarchs were not immortal

 Digganob wrote:
Way back in 2. edition, Codex: Ultramarines stated that Marines only started to show signs of aging when they were 300 years old. I don't know if that's changed since then but there's also some more recent background about Marines becoming to old to fight and "retiring":

"Although the Codex describes a number of ranks and responsibilities within the headquarters staff, only a very few of these officers actually accompany the Chapter to war. Many are non-combatants of advanced years whose roles are to find and train recruits or administrate the Chapter ..." - Codex: Space Marines, 5. edition, p. 16

Codex Grey Knights, 5e, p.24 wrote:A small few Librarians are fortunate enough to reach an age when their bodies can no longer meet the rigours of campaign. Such warriors are removed from the Chapter's fighting roster entirely and retire to the labyrinthine corridors of the Sanctum Sanctorum

Codex Blood Angels, 5e, p.12 wrote:Physically the Blood Angels are among the longest lived of all the Space Marine Chapters. One of the peculiarities of their aberrant gene-seed is that it has vastly increased the lifespan of those who bear it, so it is not uncommon for Blood Angels to live for a thousand years.

Warhammer 40,000, 6e, p.181 wrote:With their genetically enhanced bodies, Space Marines live extended lifetimes - if they do not fall in battle, they can easily live two to three times longer than a normal man, and sometimes far longer.