Well, by the end of the game I'd guess he's at least in his mid thirties. Here's why:
I'm assuming Fenris was physically completely mature when he got the markings, that is to say, 19 or older. There are a bunch of reasons I think this that I'm not going to go into at this point (if you want to come up with reasons you disagree, I'd be interested in hearing them). Then I assume a decent, chunky tenure as a slave: at least five years, possibly longer. Then three years on the run. So when you meet him, do all that little math, and he's at the very least 27, possibly older. And the game takes six plus years after you met him so... math math math.
As for maturity, I wouldn't try to correlate that directly with personality all the time. The most mature guy I've dated was significantly younger than me, but based on his personality you would easily have pegged him as mid-to-late thirties. He's actually one of my "rivalry" exes, the one who reminds me the most of Fenris.
As for the longevity of city elves, I assume that all elves age at least a bit slower than humans, but that city elves tend to die younger for all the same reasons people in poverty die younger everywhere: malnutrition, poor access to medical care, substance abuse. I don't think it's possible to know if city elves would be longer lived than humans, under ideal circumstances.