User blog comment:Supahbadmarine/User Learning Lab/@comment-9465750-20120718184204/@comment-2253300-20120718184530

A good question. I would say that depends. Warbands that are outside Warp Space tend to be constantly fighting, and being hunted by the Imperium. However while staying in the Warp is part of their longevity, another part of it can come from the gifts of the Chaos Gods themselves. I have stumbled across a number of text that mentions that the Chaos Gods granted their followers unnaturally increased lifespans. So yes, it is technically possible, though without a safe haven they are unlikely to live that long due to enemies and constant fighting.