Is death from old age contingent upon creature type or the specific creature?

Question

While I think it's great that the Monster Manual mentions which creature types breathe, eat, and sleep, it doesn't mention anything about a creature's type similarly affecting how a creature ages. By default do creatures of all types age, and—if not slain first, obviously—eventually die after exhausting their random years at the ends of their venerable lifespans? For example, does a creature with the type fey or nonnative outsider eventually die of old age?

Alternatively, is the end of existence due to old age only a factor for living creatures, constructs, deathless, and undead remaining forever perfectly preserved by their magical creation?

I am looking for official confirmation either way. Answers should refrain from speculation and suggestions for house rules until after official Wizards of the Coast sources are exhausted.

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Context: I'm worldbuilding. An ancient race has left behind ruins inhabited by protectors. I'm trying to figure out officially what kinds of creatures the ancient race could employ—with minimal alteration—that would be able to guard the race's treasures for thousands of years in complete isolation. I know that there are ways for even aging creatures to artificially extend their lifespans so that they approach infinity, but I'm not looking for such workarounds. Instead, as a DM, I want dead simple: y'know, plop a boring, everyday blue slaad or nymph into a dungeon and be able to tell the players with a straight face that the creature's been there for 10,000 years and have such a statement not violate the game's official canon.

Answer 1

There is a whole lot of undefined subject matter here, but I am confident, at least, in saying that being ageless or age-limited is not purely a function of creature type. Some creature types are probably all ageless or all age-limited, but some seem very likely to be a mix and it would thus have to depend on the individual creatures whether they are or aren’t. Unfortunately, in most of those cases, the rules won’t tell us which it is.

Aging is defined solely by age categories. If there are defined age categories for a creature, its longevity and relationship to senescence are well-defined by those categories. But they only ever define age categories for PC races, and for dragons. Perhaps a few other creatures. But there are even creatures with discrete references to age, in terms of stats for younger and elder versions of the creature, that nonetheless neglect to fully define age categories.

I think we can assume that constructs,¹ elementals, outsiders, and the undead are by-and-large ageless, just by virtue of what they are. We can assume that animals, humanoids, plants, and vermin are by-and-large age-limited, just because these are things we mostly know from real life. But I would hesitate to make blanket statements about most other creatures, and would not expect that the concept of old age is defined as a blanket characteristic of those creature types.

This leaves us to look at the individual creatures in those types, but most of those lack age categories and thus give us no insight. So to avoid alterations, you are likely looking at purely spiritual things like elementals and outsiders, non-living things like constructs or undead, or a PC race or dragon explicitly defined as ageless.

Finally, it bears mentioning that Dragon vol. 354 introduces a number of rules about ancient characters, including the Wedded to History feat. Said feat is often purported to supply a character with the ageless quality, but that is only ever implied by the feat and not stated outright (and is not, strictly speaking, required; creatures with the Wedded to History feat could instead be preposterously long-lived rather than truly ageless).

1. _{Possibly excepting living constructs; Eberron explicitly leaves the question of warforged old age undefined and up to the DM. It is one of the greater “open mysteries of Eberron.” Seriously, Wizards of the Coast took a great deal of care with the warforged aging table and it is one of my favorite things in D&D.}

Answer 2

Official position appears to suggest DM fiat as the answer.

This archived article technically deals with 3.0, since it was posted online a few months before 3.5 debuted, but I think this line is relevant: "Folklore and legends often portray fey as immortal, but the core D&D cosmology is silent on the issue of fey immortality." This explicitly acknowledges the (perhaps deliberate) silence of the D&D rules on the immortality of fey, and could reasonably be applied other fantasy creatures where the rules are silent. The article goes on to discuss at length some options a DM might consider in deciding whether and to what extend fey are immortal, and these considerations would be relevant to other types and kinds of creature than those discussed.

So far as I can see, you could just plop down your choice of fey guardians and so long as their entries do not directly or implicitly limit their lifespans you should be in the clear so far as the official D&D rules-as-written. An example of what you might want to avoid is the Tree Dependent (Su) feature of Dryads, which mentions they are "mystically bound to a single, enormous oak tree and must never stray more than 300 yards from it. Any who do become ill and die within 4d6 hours. A dryad’s oak does not radiate magic." Rules-as-written this would imply a Dryad has a limited lifespan in practice if not in theory, since a non-magical oak tree eventually dies of age.

My advice is to pick a creature that appeals to you and search through splat-books for anything that explicitly mentions their age; and, in the absence of such, you should be free to assume they are — or are not — immortal as you like.