The term Golden Age referred to a great period in history both on Earth and Bajor.
Earth
On Earth, the Golden Age was a mythical first age at which Humanity was seen to exist in a utopia.
In 2267, the crew of the USS Enterprise discovered that the myth of the Golden Age originated from a race who visited the Mediterranean region of Earth around 2700 BC. Due to their psychokinetic abilities, they became known as the Greek gods. Greek civilization and consequently, much of later Human culture and philosophy derived from the worship of these beings.
Bajor
On Bajor, the Golden Age was a prophesied time of Bajoran history that would have began after the Kosst Amojan was destroyed during the Reckoning.
After the Reckoning began in 2374, the Golden Age was not brought about, as Kai Winn Adami forced out the Prophet and the Pah-wraith. (DS9: "The Reckoning")