No, I'm not asking about how old elves or any other races get. I don't know if there's any information on this, but how do different ages age? How would a 20 year old bosmer look, like a little child? And a 20 year old imperial compared. And orcs? If some elves get hundreds of years old, but that is very rare. If I assume a typical altmer gets 150 years old, but some (non-magically) get many hundreds of years old logically a 150 year old altmer would still look young?

Barenziah was quite mature at her 20. She looked middle-aged at her 400, but become old lady 30 years later.

Err, I think the part about looking middle aged to old in a 30 year span from 400 was more to do with some more lore fine-tuning from making all elves living up to 1000 to the variation nowadays.

But from what it looks like, and considering The True Barenziah is the only source that goes into this, Barenziah was still able to make it look like she was a boy at 16, but couldn't hide it when she was 17. Also, it seems that elves become fertile at 18 or 20, as this was mentioned in book 2, and how surprised Tiber Septim was when he got her pregnant in book 3 (elves aren't supposed to be fertile till a few more years). So, it looks as though they develop physically like humans, but are delayed when it comes to making babies.

From there on, it looks like they have a really extended advlthood. At the point of when they look old depends on how well they are cared for. For a peasant, they'd look older sooner (as well as die sooner). For nobility, older later (and die older). Wizards cheat, so don't base much when it comes to elves and magic.