I have not really found anything about it in the ES lore.

But I do know that in certain legends some werewolves would live to become thousands of years old. Sometimes immortal.

Do I like to think that's the same case in Tamriel? No, not at all.

To me, lycans aren't ageless like Vampires. But they do get supernatural strength and heightened senses, aswell as becoming more athletic and active in general. And don't the forget the resistance against diseases. I believe that this might increase the person his/her lifespan because of such improvements. Not to the point where they become thousands of years old. But maybe about 20 years older than the average man, depending on the personal skill with the curse/gift.

What do you think?

Yeah I don't like the idea of lycanthropy extending ones life by hundreds or even thousands of years either. A little extra to the natural life span yes, maybe 20,30,40 years.

If we don't have in game sources, then all we can do is speculate. If I had to hazard an uneducated guess, I'd say that their life spans are the same. Kodlak seems worried about his afterlife in his old age after all.

Since we don't have a hardcoe mode, there's no information on metabolism. My guess is that lycanthropes have the same lifespan as their non-lycanthropic counterparts. They are, after all, alive. Indeed, if I had to guess, I would say that werewolves have shorter lives than normal, though they stay healthier while they do live (due to immunity to disease).

Vampires, being undead, have no metabolism aside from the need for blood. They don't need to ingest food, drink water, or sleep, and they're not subject to hypothermia either. So, they don't age, and have indefinite lifespans.

We don't know how old kodlak halftime is though.He could be 100+ years old

This...

Reading the books about past Harbingers and even talking with Kodlak seems to suggest a rough 200-300 year life span perhaps. Just a theory of mine based on the impression I got. I thought there was a book listing approx 10 harbingers over a time line dating back to Ysgramor himself. My head cannon is a werewolf lives roughly 150-175 years average give or take how they live their lives. Do they die in battle of or old age.

And I always seemed to get the impression Kodlak is not some 80 year old man contemplating death. It seems he has been around a lot longer then that.