Longest Living: Telvanni Mage-Lords
Shortest Living: Orcs (Had that curse thrown onto them instead of the Nords)
Out of the humans, Bretons live the longest, thanks to their elven blood. If you discount the Telvanni, Altmer live the longest thanks to Phynaster teaching them how to walk better.
The only known statement about the age of Bretons is not consistent with the belief that their elven blood grants them a longer lifespan. If the words in
Brief History of the Empire are taken at face value, Bretons do not live particularly long and do not age well.
"When Cassynder assumed the throne upon the death of his mother, 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, he was pressed into accepting the throne. To no one's surprise, the Emperor Cassynder's reign did not last long. In two years he joined his predecessors in eternal slumber."
Cassynder cannot have been more than fifty-nine, and probably a few years less. It is important that the historian writes "aged like a Breton", because it indicates that his early decrepitude is generally known to be characteristic of Bretons.
I cannot conclude that Bretons have a long lifespan; on the contrary, they have a short lifespan as Men go, and they are known neither for living long nor being hearty in old age.