Mordax et al

Yes, Carapaces are biologically immortal.  If their vital functions are not impaired by any outside intervention they can and will live forever. Most of the Dersites that live and work on Derse (I would say greater than 80%) were created toward the very beginning of the medium.

They need little sleep, they are resistant to temperature change, and can go long, long stretches of time without any significant food or water intake (though they will certainly not enjoy the experience).

This immortal status I have observed has had a number of unique effects on their culture and society, which, I must admit is part of the reason I have become so interested in them; and further articles will touch on these effects.

As for other beings within the medium, it varies from creature to creature. Denizens, as I understand it, are immortal by virtue of their unique status as Denizen. I have not done any great study of the denizens in fact, though several of my fellow players are now on speaking terms with theirs.

Imps, as servants of the Denizens, I am unsure of the status of. They came into existence when we entered the medium, created I surmise, by a surge of power in skaia caused by our entry, and thus they have not been alive long enough for any of us to discover if they have a natural life span. I will admit, the question had not occurred to me until you raised it.

Consorts, on the other hand, very definitely have a natural lifespan. Its length varies from species to species and I have not done anything like a comprehensive survey (my expertise, as I have said before, is on Derse), but it seems to range in length from around 30 years to around 200 in the longer lived consort races.