How old would a 80 year old Dunmer that lived a posh life look ?

Given that dunmer can in good conditions and barring magical aid live to about 200, I would imagine they would look middle-aged by human standards. Of course, from what we see in game, dunmer are naturally more wrinkled than other races, which leads to them looking older than they may actually be.

Also, as http://en.uesp.net/wiki/Lore:Neloth clearly demonstrates, magic exists to both extend one's lifespan and make one look younger.

Neloth is nothing. http://en.uesp.net/wiki/Lore:Divayth_Fyr was four thousand years old by the time he showed up in Morrowind. He was even born as a Chimer. He's an outlier, of course, but Morrowind implies that Dunmer mages tend to live a very long time, even for the species average, and even without being particularly powerful. Neloth considered the two hundred year old Senise Thendo a child, and she's a low rank in House Telvanni. She doesn't look old, either.

That would seem to be for lower class Dunmer, though. Note that Barenziah was 430 as of Morrowind; granted, she looks quite elderly but not exactly on death's doorstep.

Let's not forget http://www.uesp.net/wiki/File:SR-npc-Sondas_Drenim.jpgguy, who claims to have been in the field mining for over a century. Last time I checked, that's not exactly a cushy job that equates to longevity. He certainly is careful about combating air born illnesses though, which could explain his healthy and youthful appearance. There's also the Sarethi sisters one of whom previously lived in Vivec and the other in Mournhold, likely fled to Skyrim during the Red Year. Now one of them certainly looks like they're getting up there in years, but they're still passed two hundred year mark at least.

So my figure of 200 years wasn't best accurate. Not sure where I got that from, to be honest.

I cited Neloth not so much because of his age but rather because he looks like he's younger in Skyrim than in Morrowind, despite it being 200 years later, presumably by the use of magic.

ESO devs answering questions, IIRC. Not that its any faults of their own, each individual person who are writing for different characters, which I expect is quite a few, seem to have different ideas of how Mer age and how long they can live. So, we end up with numbers all across the board.

Personally, I roll with 300-500 hundred years being around where a Dunmer or Bosmer could be considered old, with 500 being those with significantly better living conditions.

it sounds like at least for elves, they age not from merely time passing, but from stress. it was mentioned that high elves learned by walking slower they could extend their lifespan a hundred more years. it was also mentioned that nobility lived much longer, likely attributed to less stress and worry.

I wonder if that would actually mean that a 200 year old commoner would physically look like a 400 year old noble.

Bear in mind the ESO devs are talking about ESO's era, which is set many centuries before the mainline games. The numbers may not necessarily hold up in modern times, could be advancements in medicine, improved Restoration understanding, etc. allow for even the average Mer to live a little longer than their Interregnum-era ancestors.

I do prefer elves being immortal as in tolkien's original concept. It gives them a meaningful distinction to men, as opposed to just having pointer ears and faces.

There are no mentions of elves living forever (not without magic, anyway), but the Morrowind pre-release item "http://en.uesp.net/wiki/Morrowind:Interview_with_a_Dark_Elf" mentions a natural one thousand year lifespan, but also mentions that they tend to die from violence or disease before they can die of old age. The interview is almost certainly non-canon so the info in it is questionable, but a thousand year lifespan under ideal non-magical circumstances could work out, with magic being able to extend that further.

Even with that they are more mundane than the Middle Earth elves, who were immune to illness.

Except there's not really meant to be such meaningful distinctions. Men and Mer come from the same Ehlnofey ancestors, Mer are just the ones that held onto their biological heritage more, so they still have the facial features and longer lifespans of the Ehlnofey. The distinctions are in cultural mindsets more than physical traits.

Of course, even Tolkien's Elves have some caveats to their immortality: first, it's bound up with the existence of the World and their fate after that is unknown; second, their spirits burn so hot that they consume their bodies, causing Elves to ultimately become disembodied spirits. And of course they can die from grievous injuries or from world weariness. On the other hand, they can also occasionally reincarnate, though Glorfindel is the only explicit case of this.

Barenziah certainly aged badly between Daggerfall and Morrowind, didn't she? It was only twenty years, too.

Well, technically they all reincarnate in Aman, and Glorfindel is the only one to return to ME.

She also had brown skin and blue eyes in Daggerfall and only switched to a proper Dunmer appearance with the a fan patch. Neloth also looks younger in Dragonborn than he did in Morrowind despite being two hundred years older. I generally just chalk up changes in appearance to changes in art style.

Some: not all leave the Halls of Mandos.