Extremely old. When someone says they are "old as dirt", they are comparing themself to the earth, which has been around for basically forever. Depending upon your religious opinion, either since God created it or just after the big bang happened. Eitherway, the Earth has been around, with dirt on it, for an extremely long time. Thus, "old as dirt" would make someone extremely old.
My grandfather is "old as dirt".
ayon kay Skyon Archer ika-05 ng Hunyo, 2012