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.
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.