Just a quick question: What is the average lifespan of mer?

its all very inconsistent with the information we are given. Many people say that Altmer can live for another 1000 years than other elves, yet Barzeniah(spelt correctly?) lived for i think more than a thousand years or more implying that elves can live maybe for more than 1000 years or more in some cases. But many people on these forums say that elves can only live for a few hundred years and that Altmer living for more than a thousand year is exhaggerated and they just live slightly longer than most elves.

Barenziah is only about 450. you may be thinking of Fyr.

And Telvanni wizards, and elvish wizards in general, tend to be the exception to the rules of age, due to the magicka they will use to extend their lives.

Peasent=150 to 250 years.
Nobleman=Around 500
Sorcerer= about 1000 years even more if your a telvanni.

Quick answer: It's disputed.

The Real Barenziah mentions a lifespan of a thousand years¹, and various NPCs mention the dangers in dealing with thousand-year-old wizards². I suspect the passage in The Real Barenziah is poetic license and does not intend to make a definite statement about lifespans, but the wizards who live a thousand or several thousands of years are well attested. These are exceptional cases, as is the long-lived Barenziah.

The secondary source Interview With a Dunmer suggests that a lifespan of 120 to 200 years is not unusual among the Dunmer, barring accident or disease³, so this is often taken as a best guess. This implies that the lifespan of Dunmer has a long-tailed distribution, with a non-trivial number of them able to live to extreme age (by sheltered life, access to the best healers, necromantic means, or the teachings of the Psijic Order).

Altmer make much of being longer-lived than other mer, attributing this to their ancestor-god Phynaster, who taught them how to live 100 years longer⁴. The exactness of this is open to question, but an assumption of an average lifespan of 200 to 300 years or more for Altmer would not be out of order. Also, it is not unlikely that the distribution of Altmer lifespans is long-tailed, with some number of them able to live to extreme age.

¹http://www.imperial-library.info/dfbooks/b101_complete_barenziah.shtml#4
²Caius Cosades, in Morrowind
³http://www.imperial-library.info/interviews/alvur.shtml
⁴http://www.imperial-library.info/mwbooks/varietiesoffaith.shtml

I'd say with the 3rd PGE seeming to imply that a large number of Altmer have first-hand memory of Summerset's surrender, you can safely assume that Altmer may average more around 400-500 years, if not longer.

Considering the Real Barenziah's 1000 years come from Daggerfall and all later sources counter it when it comes to 'normal' elves, it can be ignored. Thousand year life spans only apply to mages, including the Altmer because Phynasters 100 years wouldn't all that impressive enough to make him a hero-god.

How old is Barenziah herself?

Ahem.