The Age of Harmony is the name given to the period between Vardalon's creation and the cataclysmic events of the Sundering. The exact duration of the Age of Harmony isn't known, but it seems to have been extensive enough to allow the formation of entire cities and cultures. Conservative estimates seem to indicate between 3,000 and 4,000 years, but others have been known to assign 10,000 years or more.
The Age of Harmony is a retronym given in light of the current Dark Age. Compared to the life the people of Vardalon know now, the thinking goes, how could the time before the Sundering be anything but better. Certainly collected writings seem to indicate this; war seems to have been a non-entity, and there are no extant texts that mention sickness.
More will be revealed about the Age of Harmony as the game progresses.