What's the comparison human to High elves in age? like how old is a High elf looking like a 45 aged human?

Wild guess. 45-400?
I can't remember how old they get, but it's really old. But I think they look the same untim they reach advlthood and then the advlthood lasts for a looong time until they are old a long time... If that made any sense. Say 0-20 is like 0-20 for men, but 20-40 for men is like say 20-400. (random numbers)
But then again, I am basing this on next to nothing.

why you say this then?

A random thought that I think might point you in the right direction until someone came up with something better.

I don't think that in any game i've ever read something about the age span of the Altmer. And i've played since Arena. It doesn't make sense to me that they live for THAT long. I could be wrong tho >_<

They mature at the roughly same rate so an 18-year old high elf will look the same age as an 18-year old human. Once advlt their aging will slow down considerably though. The high elves live to be up to about a thousand, depending on their social status and access to advanced healthcare they'll be above or below that number, a noble or mage-lord could surpass that while a peasant would have significantly less than that.
I also suspect that they don't have quite the same level of infirmity in old age so an elf will never look as old as a very old human.

Simply put: we don't know. There are very few npcs with detailed biography out there and most npcs simply don't die out of age/illness like Nurelion does.

Barenziah was 400+ at the time of Tribunal, I think it's a good start as I don't see why altmer et dunmer should have different lifespan.

Depend on social status. Consider dunmer commoners usually live up to 160-200 years, and altmer commoner should best that due to better environment and their Phynaster steps, extending their lives 10% iirc. So 200-300 years for a commoner it seems.

In the Real Barenziah it states that there are elves live up to 1000 years but most never do. Those must be great mage lords and scholars. General nobles and mages are in between, longer than 200-300 but not reaching 1000. Longer lives are like the case of Divayth Fyr, pure magic and not in general discuss.

High elves are longer lived than other elven races.

Average dunmer live to be about 150. Bran-shei is about 195 and is less than middle aged, but that's probably because of his bloodline.

I'd assume that most altmer live to be about 600 or so (because of Phynaster and their culture's value of racial purity). Altmer living to be a millenium isn't that outrageous, but I'd say it's not common.

In De Rerum Dirennis , Vorian Direnni is 611 and he is old. And in the same book, it states Medora Direnni (advlt and active in 3E 417) has her grandfather Jovron Direnni (advlt and active at least from 3E 154- 3E 200). That is about 260 years. And Jovron has his great-great-grandfather Pelladil Direnni (advlt and active at least around 2E 230). That is about 820 years. So in averange the Dirennis takes about 160 years to generate a new generation, and it seems ok to assume each generation can live to 500-600 years. (Just like a noble man takes 15-20 years to have his own child and he can live to at least 50-60)

But the Dirennis are noble Altmers. Consider the differences between the lifespan of a commoner Dunmer to a noble Dunmer, which is about 160 to 400-500 (Barenziah is over 400 and she is considered old). If the ration is the same, as all commoners live harder and more dangerous lives than nobles, then we will have the Altmer commoners live to about 200 years. Then we plus the magical affirnity, pure blood and better living environment of Summerset Isles than Morrowind, and the 10% from Phynaster steps(20-100 years) we can extend the commoner lifespan to at least 300 years, but of course never reach 500-600 years of noble lifespan.

Skyrim has the most explicit references to lifespans of Dunmer, I think, with several Dunmer commoner characters who were born before Red Year and therefore approach or exceed 200 years of age. Beyond that, all the info we have has been given in this thread.