Which (if any) fiends die of old age?

Question

I've seen people say that fiends can live forever, but never a source for where this fact is from or when/why it's the case. At least in 3.5e various things refer to creatures' "time being up" without mention of exceptions, but is there any reference in any version of the game that points one way or the other for extraplanar/native demons/devils?

Answer 1

Devils are immortal

In 5E, the Monster Manual entry on Devils has this to say:

Dark Dealers and Soul Mongers. Devils are confined to the Lower Planes, but they can travel beyond those planes by way of portals or powerful summoning magic. They love to strike bargains with mortals seeking to gain some benefit or prize, but a mortal making such a bargain must be wary. Devils are crafty negotiators and positively ruthless at enforcing the terms of an agreement. Moreover, a contract with even the lowliest devil is enforced by Asmodeus’s will. Any mortal creature that breaks such a contract instantly forfeits its soul, which is spirited away to the Nine Hells.

To own a creature’s soul is to have absolute control over that creature, and most devils accept no other currency in exchange for the fiendish power and boons they can provide. A soul is usually forfeited when a mortal dies naturally, for devils are immortal and can wait years for a contract to play out. If a contract allows a devil to claim a mortal’s soul before death, it can instantly return to the Nine Hells with the soul in its possession. Only divine intervention can release a soul after a devil has claimed it.

Demons are trickier

If you destroy a demon outside of the Abyss, it simply reforms there. If the demon has crafted an amulet, even destruction in the Abyss is not the end. The Monster Manual makes no mention of aging or natural death. However Demons are agents of chaos and destruction which endlessly fight with everything including each other. The things they don't attack are stronger demons they are forced to serve.

It's reasonable that Demons do not die of natural causes, but if you meet an ancient demon, it's safe to assume that it is very powerful to have survived that long.

Hags are far from mortal

The Monster Manual says so in the opening line: (emphasis mine)

Hags represent all that is evil and cruel. Though they resemble withered crones, there is nothing mortal about these monstrous creatures, whose forms reflect only the wickedness in their hearts.

They can eat children and give birth, but again, there seems to be no limit to their existence, timewise. They are called ancient.

In Conclusion

We start to consider what it means to be immortal. It is the inverse of being mortal. It's reasonable to assume things from beyond the mortal realm will be immortal.