I've been doing a bit of in game reading and I remember the phrase 'he aged like a breton' in reference to the half-elven emperor whose name I can't remember. So presumeably Bretons have an age span that is intermediate between that of elves and those on the more human races, given that thier racial is extra magicka that would seem to support the thoery that is it a person's magicka 'store' that allows them to live longer, provided no outside factors prevent it.
Obviously a person's birthsign would also affect thier lifespan, with atronachs possibly living the longest under 'natural' conditions.
The "aged like a Breton" line refers to Cassynder, and the sense and context it's used in make it disparaging.
"When Cassynder became Emperor at the death of his mother, Empress Katariah, in 3E 200, he was already middle-aged. Only half-elven, he aged like a Breton. In fact, he had left the rule of Wayrest to his half-brother Uriel due to poor health. Nevertheless, as the only true blood relation of Pelagius, and thus Tiber Septim, he was pressed into accepting the throne. His rule was a short as he died after only two years on the throne in 3E 202 which was assumed by his half-brother, Uriel Lariat (Uriel IV)."
So he was middle-aged by standards such as are applied to half-elves and Bretons at the time. He can't have been older than 59, as Katariah had married Pelagius III in 3E141. "Aged like a Breton" is clearly disparaging, as "in fact, he had left the rule of Wayrest... due to poor health" immediately follows and explains the phrase and his condition.
By the only example we are given, there is no other conclusion to draw than that
Bretons are not long-lived and are known for not being hale in old age.