From the White Wolf Wiki, the most complete White Wolf reference

The First Age is, as the name implies, the first age of the Wheel of Ages. It was the Golden Age after the Exalted defeated the Primordials, when wonders and magic undreamed of in the Second Age were commonplace. The First Age is commonly considered to stretch from the end of the Primordial War to the time of the Great Contagion, nearly eight centuries ago. Most learned savants place the fall of the First Age some 600 years earlier, however, with the murder of the Solar Exalted.