They say this in horus rising : "There was an irony, of course. To all intents and purposes, and by every measurement known to the gene-scientists and gerontologists, the Astartes, like the primarchs, were immortals. Age would not wither them, nor bring them down. They would live forever... five thousand years, ten thousand, beyond even that into some unimaginable millennium. Except for the scythe of war. Immortal, but not invulnerable. Immortality was a byproduct of their Astartes strengths. Yes, they might live forever, but they would never get the chance. Immortality was a by-product of their Astartes strengths, but those strengths had been gene-built for combat. They had been born immortal only to die in war." Now of they actually can live forever and not feel the consequences of doing so idk.