So does anyone know the expected age of a Breton? Seeing as they are man-mer I would guess they would live longer than "normal" humans, but is there a resource that gives an actual answer?

Cassynder Septim is said to have "aged like a Breton". That the Cyrodils should have such an idiom indicates that they aren't long-for-mundus.

That's kind of interesting. Elves are supposed to live a long time, but half-elves (like Cassynder) and Bretons age more rapidly. I wonder why?

I'm not sure it indicates that he aged any more rapidly than an average human. The whole whole quote says that he was only half-elven, and hence aged like a Breton. Considering that it goes on to mention failing health, it may mean that Bretons age poorly? Either way, I don't think that one snippet is enough to determine Breton lifespan for certain.

I agree, it simply seems to vague.

Mundrial metabolic activity produces free-sithical errata, low-order planetaculii nonetheless oblivirious to the delicate filigree of adabalhelices-a from which all mundroids - Merrimorphs, Mannoids, even Histologues - derive their moment-to-moment hypostates. This is the phenom known as aging.

Merrimorphs derive their longevity from passive thaumanutritive supplementation, effectively obviating a large portion of their physical dietary intake, and therefore the associated hypostatical dommage.

The apparent dearth of moons and winters seen by Merromannish hybrids is due to poor, or rather, incompatible lifestyles - Bretons and their ilk attempting to emulate the shezarrisithical gourmandizing of their fellow mannoids proves too "rich" for their mixed blood.

It could as easily mean that he aged slower than other men but the sicknesses took him young.

Really, we have no bloody clue. While we know for a fact that certain elves are far older than human spans (Sarethi from Skyrim, Barenziah) beyond that we know no reliable specifics for any race.

Even though you're making up words, I can actually understand what you are saying. I usually just skim over your freaky monkeytruth, but I now realize that it's not just nonsense.

Bretons aren't half and half but rather they are humans with some elven genes. They are to my knowledge most part human though the elven input has had a greater impact upon their acceptance to magic.

Most Bretons throughout the ages have reproduced amongst themselves with other Bretons.

It is throughout the time of Elven rule, that some Breton women that were used as six partners / slaves by the elves. It is through these that the Elven genes have been absorbed. To my understanding it is far from half and half though hence their human appearance.

I am aware of that, but tha fact that one of the Septim Emperors were said to "age like a breton" implies they age differently than other human races.

Isn't that quote comparing him to his mother, Katariah, though? That merely suggests that they live shorter than elves, which is no suprise.

Basicly a child belongs to the race of the mother, so Cassynder Katariah's son should age like a dunmer, but in fact he aged like a human, a breton, as he was the son of Pelagius III.

Yet this always puzzles me that it seems Pelagius was a Breton, yet his mother was from Direnni Clan. So were Utheilla Direnni and her daughter who later ruled Solitude both Breton, or any children born from a Septim and an elf would always be more manish than elven? Or Pelagius III was in fact son of Hellena who we know so little but seemed to be Cyrodiil or Breton?

Clan Direnni have ruled in High Rock for millennia. It is very likely that some parts of the Clan would not be pure-breed Altmer, in fact, it is difficult to imagine any of the Direnni to be purely merish by the time of Utheilla.

That said, there are no hard laws of Elder Scrolls genealogy. Even the "child resembling mother" one is merely rule of thumb. There are exceptions.

Then despite of the mages and wizards, like Vorian Direnni who had lived at least 611 years, there were some Direnni who were more Breton than Altmer that they were born to live much shorter lives than their Altmer relatives.
And I find in the thread On Boethiah's Summoning Day there mentioned some Bretonish Direnni who were resisting Thalmor, and their leader was Balfiera's Castellan, whom I believe was Medora Direnni according to the year. I don't know if this thread is included into semi-official lores. A mix-breed herself she seemed to be, or she was a leader of both a mix-breed part of the Clan and the whole Clan, due to these Bretonish-Altmer's relation to the Septim Dynasty?
Never knew their fates in the following years though.

I don't take "Boethiah's Summoning Day" to be canonical lore. It is monkey truth, and fan-made monkey truth as that. Fun but nothing to draw upon. I'm not sure that Modera Direnni should be taken as mixed race, considering a number of full-blooded Dunmer in Daggerfall looked somewhere between Bosmeri and human. If anyone has any lore suggesting that the leaders of Clan Direnni were half-breed, I'd be fascinated to see it. One possible indicator could be the amount of generations given in De Rerum Direnni, which is excessive knowing the possible life-span of a well-bred High Elf. Personally I put that down to a lack of imagination in the writer's part, though who knows?