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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We know of several Eliksni who lived pre-Whirlwind. Variks and Craask are two others. Craask seems to have been in a leadership role of some kind before the Whirlwind, and is the only Fallen I recall being described as old and weak, having been Kell of his House for centuries, presumably. I can’t recall the source, but I vaguely remember Saladin as being nearly 1000 years old. If that’s the case, Craask almost certainly lived for more than that, before he was killed by Fikrul.