Do non-native outsiders die of old age?

Question

Outsiders are described so:

Unlike most other living creatures, an outsider does not have a dual nature—its soul and body form one unit.

The bond between Outsiders and their Home Planes is strong, if they are slain in the Prime Material Plane (or any another plane in which they gain the extraplanar subtype) they come back in their original plane.

For example, once a Devil is killed (Fiendish Codex II, p. 18) this happens:

A devil slain in the Nine Hells stays dead. A devil slain outside Baator devolves into a puddle of foamy, stinking ooze over a period of 3 to 9 minutes. This residual soul essence registers as both magical and evil. [...]

Whether or not its residue is disturbed, a slain devil returns to Baator 99 years later, in its original form, at full hit points.

A similar thing happens to Demons (Fiendish Codex I, p. 9):

Outside the Abyss: If a demon is killed on another plane, its body eventually returns to the Abyss—unless trapped through magical means, such as a dimensional anchor spell. [...]

Within the Abyss: If a demon is killed while within the Abyss, it is permanently destroyed—both its body and its essence.

Even Aasimon have a similar way to reincarnate whithin some decades after being slain (Monstrous Compendium: Outer Planes Appendix, 1991)

However, the only way to end those beings is to destroy them on their original planes and with a violent death, if not killed they are virtually immortal.

This brings me to some questions:

1. Does this rule apply to ALL the Outsider out there?
2. Is it possible that some kind of non-native outsider could die of old age?
3. If not, does this mean that every outsider is immune to aging?

Answer 1

Most outsiders don't appear to age, but there's no general rule.

The rules for Outsider Type in D&D 3.5 are listed in the Monster Manual, p.313. They don't mention anything about outsiders in general being immortal or immune to aging. The half-celestial description refers to the celestial parent as "immortal", but there's no other reference to it, and the only rules reference I can find to "immortal" in the D20 SRD is in the deity rules.

By the core rules, this means some outsiders may age and may die of old age. Outsider immortality and immunity to aging is not a general rule, at least not by the D&D 3.5 core rulebooks.

Previous sourcebooks are vague. For example, AD&D 2e's Faces of Evil: The Fiends, p.6, notes, specifically talking about fiends:

'Course, fiends aren't invulnerable; they can be killed just like any other creature. But are they truly immortal otherwise? Does their evil just keep on raging unless their lives are unnaturally halted?

How can I know, berk? How can anyone? We won't be around to see if any fiends make it from the first breath of the multiverse to the last. The powers [i.e. deities] might know the answer to the question, but they're not telling. [...]

On the other hand, most folks assume that fiends are, indeed, immortal. It's the safest guess, and besides, no one's ever seen a fiend grow ol or die of "natural causes". See, the fiends may grow, but they don't grow older — that is, they don't mature.

The great lifespans attributed to certain extraplanar beings and a lack of age categories listed for them certainly suggests that they're immortal, and I can't think of examples of creatures which explicitly age the way humanoids do, but there's no specific rule in D&D 3.5.

There are specific examples of outsiders which do age and die of old age. The tojanda (MM p.244) lives up to 150 years, and has the outsider type with the extraplanar subtype, so it's non-native.

According to the AD&D sourcebook Warriors of Heaven, p.90, aasimon and archons do not age, but eladrin and guardianals do. For example, the ghaele eladrin reach venerable age at 400 and can live up to 800 years, and the avoral guardinal becomes venerable at 225 and can live up to 275 years. They accumulate aging penalties and bonuses as normal. Asuras can live up to 350 years, but do not appear in the aging effects, suggesting they have a finite lifespan but do not age.