I did a quick search for "Old Kingdom" in Books 1-16. Ignoring stuff from the
rules sections of Books 13-16, I found two separate references:
10tdot 298: "During the Age of the Old Kingdoms, when the Elder Magi ruled the
land, this temple was a place of learning and experimentation."
13tplor 18, 138, 333, 335: "This domed chamber is decorated with tapestries
which depict Cenerese legends, ignoble and infamous deeds that were perpetuated
during the Age of the Old Kingdoms, when, for nearly a thousand years, the
Cenerese tyrannized Northern Magnamund."
As David says, then, its misuse in TDoI is really very apparent.