They also aren't entirely sure if normal Space Marines suffer from the perils of withered age.
Blood angels are specifically mentioned as being longer lived than space marines of other chapters (3rd BA codex iirc), and it certainly isn't because their preference for getting close and personal gives them a long life expectancy. And Dante is one of the oldest marines alive 'today', being over a millenia old. So I'd say we do have evidence that space marines in general die from 'old age'.
Whether that applies at all to primarchs is subject to much debate.
On Alpharius-Omegon's 'death':
On the one hand, maybe Guilliman did outfox Alpharius, and kill him. On the other hand, maybe A-O fooled Guilliman into thinking he did. The second would be perfectly appropriate, but even a master trickster gets tricked sometimes. I think this is probably undecidable. Its even possible Alpharius let Guilliman outfox him to teach Guilliman the value of stealth and trickery (given Guilliman did depart from what seems to have been considered 'codex' methodology for Eskador) - in death Alpharius won the ideological battle and made Guilliman concede as argued above me. This could have been quite intentional. (Especially if Alpharius thought that it was futile to continue struggling given the emperor's victory et al).
Now, even with one of A-O dying, that still leaves the other one running around. We know the other primarchs knew *nothing* about Omegon with the (possible?) exception of Horus, who clearly isn't speaking at this point. He may be leading the alpha legion. Even if the survivor is O, he may be claiming to be A. Who can possibly know at this point except him, assuming he still lives (hasn't died of old age, survived his twins death, etc...).
I think i'm mostly sad that, given names like Alpharius and Omegon, he was the last primarch found. Shouldn't he be the first found and the last to die? 'I am the alpha and the omega' and all that jazz? Someone really dropped the ball here. (Also, he should really be a more knowledge-oriented primarch on par with the seers. Like the phrase 'knowledge of good and evil' coming from an egyptian phrase meaning knowledge of everything, alpha-omega is like greek shorthand for everything. GW needs to work on their symbolism.)
