The How old is he, really? question bugs me, too.

My current favored solution is for the Dirt Nap to have been a restless sleep, that he was in some way linked to Ye Zun, and whenever YZ got restless/more energy/whatever SW would awaken only enough to re-suppress YZ, then it's back to naptime. And that this happened a BUNCH of times, like 10-20, enough for SW to have a real sense that time is dragging by him, but not enough for him to have a Methos-like sense of history and loss.

He could also have had periods, every few thousand years or so, when he had to spend significant time awake and moving around--the ruins around YZ's Pillar seem to belong to several different periods and even cultures.