Eramis and Taniks were both alive back during the Fallen's Golden Age, and both seemed to still be in their physical prime when we killed them (Taniks before he lost is original body).
Given that the Fallen's Golden Age was at least several centuries ago, this can only mean the Fallen have an extremely long lifespan.
Awoken live a long time, but goddamn the Fallen live a lot longer.
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Right? Eramis, craask, variks, taniks... this list goes on and on. I assume its the ether that keeps them this way, but it could be their biology.