The Istari are themselves Maiar, the same type of being which Sauron is/was. They are the servants of the Valar. As Sauron and the Balrogs show, Maiar can and do go over to the Shadow. Sauron is really just the Shadow of the original Shadow, Morgoth. They are immortal for all intent and purposes, although they can diminish in power much as the Elves do. Once the Fourth Age begins, I will venture that the remaining Istari will dwindle as do the remaining Elves. Arwen is little more than a spirit of place, a few centuries after the War of the Ring. I imagine a similar fate awaits the Istari, but they could prove troublesome. They might be more like Men than Elves, in the end. Saruman certainly came to be, and proved mortal after his fall.
“In our every deliberation, we must consider the impact of our decisions on the next seven generations.”
-- Great Law of the Iroquois Confederacy