For a woman of 50, Delphine looks pretty youthful. Plus, elves have longer lifespans, so I think that bretons would actually live a bit longer than their fellow races of men. Not as long as full elves, though.

Bretons are generally longer-lived, not short. Not as long as Mer, but they are half-Mer so they do have some longevity in them.

Yes, and not because they are mixed blood but because they are practically human, and live no longer than. Elven blood is strongest amongst nobles, and even they are far removed from the Direnni.

There was a Septim who had a Dunmer mother and a Breton Father. Although being a Dunmer himself, he is noted to have only lived as long as a Breton instead of the centuries-long lifespan that is average for a Dunmer.

So it would seem that mixed blood has a varying result based on bloodline. An elven mother and human father produces an elf that has a reduced lifespan. The opposite, human mother and elven father, would produce a human that has a longer-than-normal lifespan.

Bretons are classified as humans, which makes sense considering they were probably the result of many generations of elven-owned Nedic slaves, so they would have long lifespans for a human, but short lifespans for an elf.

I always thought "aged like a breton" meant that they look young despite old age or vice versa. It's a really vague line though.

Me too, cause Delphine looks young for a 50-60 year old woman who was in the Great War.

No, not that we know of. The quote that gets passed around ('aged like a breton') is really misunderstood without context. It's a completely neutral statement, noting that the Dunmer emperor Cassynder aged as a Breton would, rather than a Dunmer. They're not implying anything but the fact that Breton lives are a lot shorter than elven lives. The quote exactly:

Cassynder died 2-3 years after his claim of the throne, which means he both appeared middle-aged and died in his fifties, despite being a Dunmer. In this context, "aged like a Breton" seems to mean that he aged the way a Breton would have instead of a Dunmer, because he was only part-elven like one. Mixing the blood of human and elf dirties the normally long lifespan of elves to something more human-like.

If being of mixed blood, but outwardly human does make a human have a lifespan more like an elf's, then it would make sense for Delphine to appear relatively young for a human even though she is middle-aged. Being a half-breed puts one's lifespan somewhere in-between.

Well he also had poor health.

Well, I think this is more a result of Skyrim's lack of an age slider though. Ulfric has to be around 50 too, to make his timeline make sense. Even if he joined the Imperial army at 16, he'd be 46 now. He joined when the Great War broke out, according to himself. And Ulfric doesn't look very old either, despite being a Nord. http://www.uesp.net/wiki/File:SR-npc-Ulfric_Stormcloak_02.jpg

In general, I would say that Bretons probably age similarly to most other humans, but I don't think we have anything to make a good case either way.