There are some spells in Arcana Unearthed that deal with reducing a character's age or granting immortality. (Immortal in the sense, that the character does not die of old age. I imagine it would be easy enough to come up with a spell that restores a character to a youthful condition.)
I felt puzzled that the anti-aging magics were left out of 3rd edition, as anti-aging magics would probably be one of the things that most societies would research. (Consider the quest for immortality in historical cultures, especially the medieval alchemists.) Of course, I suspect that an immortal character would have a fair share of responsibilities.
Of course, since the current "default" cosmology has the souls of dead mortals transformed into petitioners, which can become outsiders, I suppose that this could explain some of the rationale. I preferred having outsiders arise from the planes themselves, or being separate from mortals. It made them more mysterious and other worldly. The default cosmology may have solved the "Where do the souls go?" issue, but not in a paricularly interesting way, IMHO. (For some belief systems, this model does NOT work. See some of the real world belief systems modeled in Deities and Demigods.) Of course, DMs are free to ignore the "Great Wheel" cosmology which is now only one of many possible cosmologies.