The Golden Age was nearly a century of peace following the end of The Age of the Paladins and the chaos from that period. The Paladins would disband, with no need for a group of warriors when there were no threats, giving the Realm to the Magistrate. Though this age of peace couldn't last forever, as a new discovery would soon cause massive changes to the Realm.
This information comes from this stream and the A Realm Divided lore cinematic.