The Fifth Age is the current Age in the Wheel of Ages.
To the Kuei-jin, the current age is the Fifth Age of the Great Cycle or the Age of Darkness. The Chi'n Ta speak of the current world as dominated by the "Stone People", who have come from far away to bring the Middle Kingdom under their control. The Hengeyokai call it the Age of Shadows. The Hsien speak of the Age of Tribulation, where the shen have to prove themselves worthy of Heaven or be destroyed once the Demon Emperor rules. Nearly all shen agree that the trigger for the Fifth Age was the intrusion of westerners into the Middle Kingdom.