I did a general search for this, but couldn't find a definitive answer. Ie- the previous posts I found had some people saying one way or another.

Do the mer reach physical maturity at the same time as men, then their aging slows down considerably? Or is it the opposite way around and they reach maturity much later as well?

Any help on this subject would be much appreciated. I just started RPing an Altmer character and want to be accurate with what her age for a relatively young advlt should be.

They mature at the same rate, just age slower. Aging and growing are two different processes. Barenziah was considered to be an advlt at around 17, but wasn't an old woman until her late 400s.

Well I believe at 17 she was considered fertile. So it is probably roughly the same but Mer are just a little slower to mature. At least that is what I think.

They die a horrific/honorable death by my hands. Tis a mercy killing, I assure you.

Serves those knife-eared, gold/ash skinned, gaunt-faced elves right!

No, it is Man, hiding behind the Early Beard that will be slain on their day of maturity!

The only thing we know about that is that young Dunmer women tend to be sixually promiscuous. That doesn't imply immaturity across the board of merkind.