...Or, to be more accurate, one does not simply gain eternal life in 5e. At least, outside of using wish to attempt to achieve it, there doesn't seem to be a way (yet?) to ultimately prevent death from simply running out of time.
Why is this? What purpose do you think the deliberate design choice to make living beyond one's natural time impossible serves?
What is old age, to be so absolute that circumventing it is nearly beyond the bounds of reach?
In your games, would you allow a mortal to become immortal, and if so, how would you let them achieve it?
I railed against character death as a kid but not anymore Death from age and death from the blade make our characters vulnerable. It also allows people to start new chapters in gaming. Lastly, if there is some means of longevity in a campaign, it will be very special.
I do think that fantasy should allow for the quest for immortality but that it should not be a standardized given. Look back to AD&D and we see risky potions of longevity and lichdom! As options. If it was easy, there wouldn't be many demiliches.
However, a quest to live forever undertaken by high level pcs sounds cool, even cooler if there is at least a small chance of success!
Personally i like the idea that you need to drain something elses life to stay alive.
It starts off small, you gain years by killing a single kobold. Something that helps everyone.
Then, it get's harder, you must kill sacrifice a whole cave of kobolds, or a few orcs.
Then it takes a village of orcs, or a dragon.
Then you need a horde of dragons, perhaps even a demi-god.
At what point do you stop becomming the hero, and start becomming the monster?
Clone specificly allows for younger versions.
True polymorph into a younger version of yourself could work too. But you might have to level up again.
It is definitely possible for characters to achieve immortality within the rules of the game. I filled a whole novel (well, Part 1, anyway, I never finished it) with perils the pitfalls of a bunch of 5e characters trying to achieve immortality and, in some cases, succeeding. The twist was, that when they got it and realised the implications, they weren't sure they wanted it any more but by then it was too late. https://drive.google.com/open?id=0Bx9fMEh0tr21ellTTlhLemJEenc&authuser=0
Was I the only one that saw DMG p. 232, Boon of Immortality?
In the PHB, there doesn't appear to be any way to do this short of Wish, true. (Clone is immortality by forking, which isn't quite the same).
In the DMG, though, there's an Epic Boon that explicitly gives you immortality. ( EDIT: Scooped by Aaron.) There's also the Potion of Longevity, which shaves several years off of your age but leaves the rest of you alone, although it's a bit of a gamble (each one after the first has a cumulative 10% chance of actually aging you a few years instead, so by the time you swig your 11th, you're effectively always drinking potions Potions of Decrepification.)
There was an article, written by Hal Maclean, called 'Ancient PCs: Playing Elders in D&D" (Dragon Magazine #354) that I really enjoyed.
What if a PC had lived for a thousand years or more? How they became an Elder could be explained several ways: maybe the PC was catapulted forward in time somehow, or maybe frozen in time.
Ancient backrounds are explored: The PC might be the last apostle of a forgotten god (and a tiny fragment of the god's power keeps the character alive through the centuries). The PC might be a 'golden ager' or a 'throwback', born long, long ago, when the world (and magic) was quite different. The PC might simply be a savage survivor, ripping and clawing her way through history.
There are guidelines on how to handle ancient monsters. Some ancient feats round out the article. The capstone is a special quality called endless, where the PC basically ceases to age past a certain point.
The article is written for v3.5 but the ideas translate readily enough. Figured it would be of interest and wanted to recommend it.
Artifact wrote:
AaronOfBarbaria wrote:
Are there actually rules for old age in 5e? I don't remember seeing any, but they say the memory is the first to... what were we talking about?
ChrisTheS wrote:
mellored wrote:
At Monk15 - Timeless Body - you still can die of old age, but you stay young and strong in the meanwhile, so you can put all your vigor and vim into finding a way to cheat death.
(Open my sig; one of my characters has already thought of another path.)
Well, the monk's Timeless Body, as mentioned above, gives some boost in vitality.
The druid's version of TB effectively boosts remaining lifespan by 10 times, too.
I'd have been truly content if these two were merged into one TB, though...
mellored wrote:
I'd argue that Timeless Body could do it. Old age isn't a thing that kills you, it's something that causes effects that lead to death.
If a Monk ignores the effects of old age then surely they couldn't be killed by it. 
But I'm just playing Devil's advocate. It would be pretty ridiculous if all experienced monks were immortal.
mellored wrote:
Lord_Avalanche wrote:
The main idea here is thematic, I think. It is a deep and abiding trope in fantasy literature, myth, and legend that the desire to transcend one's limits (not, not achieve axcellence within one's limits, but to bypass them) is basically a bad thing, especially if this is attempted by one's own efforts and not as a result of a divine boon. Mortals are, well, mortal, and the best way to live a well-lived mortal life is to accomplish one's life from within those limits. True, in some myths, mortals can achieve actual immortality (e.g. many of the heroes of Greek myth, or the quasi-immortality of Arthur on the Isle of Avalon, etc.), but even divinely-granted immortality can be perilous when it comes from asking/overreaching as opposed to an unsought for gift (e.g. when the goddess Eos asks a boon of immortality for her mortal lover Tithonus, only to realize she forgot to ask for him to be eternally young as well; he simply continues to age and get more and more feeble, in some versions finally becoming a cricket and begging for his death).
So, the basic idea is that immortality which comes as a free gift because of heroism, virtue, etc. (i.e. when one does not seek immortality) can have a place in the "source material", whereas the attempt to "cheat" death on one's own terms is almost always morally perilous and usually leads to corruption, disaster, or both.