Oldest fungus?

[RushWayne]

We have all read about the fungi that are single 
organisms and have spread over vast tracts of
land, vying for the title of largest living
organism.  I'm wondering if anyone has an estimate
of how old these giant organisms must be to have
spread over so much territory.  Perhaps they are
among the oldest living organisms, in addition to
the largest. This information in turn might have
some bearing on the issue of fungal senescence.
--Rush
http://www.mycomasters.com/
"Growing Mushrooms with Hydrogen Peroxide"