Personally, I'd venture that they live for as long as they are useful to the Hist. If the trees can change the Argonian's gender and form, then they can probably change their lifespan as well.
This is probably true, but I doubt the Hist are interfering with the
average Argonians lifespans. I would think the Hist altering ones lifespan would be quite extraordinary. Almost like the Hist version of Divine intervention.