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