The Mythic Age is the period of time between the fall of Rome and the Renaissance, the era before the rise of the Technocracy calcified the Consensus. The Mythic Age roughly corresponds with the Middle Ages in Europe, and thus the High Medieval era is referred to as the "High Mythic Age" by some mages.
Some mages (mainly Hermetics) track the start of the Mythic Age to the founding of the Order of Hermes in 731. The Mythic Age roughly overlaps with the Third Age as reckoned by the Celestial Chorus, which begins with the Council of Nicaea (325) and ends with the Declaration of the White Tower and the foundation of the Order of Reason (1325).
The Mythic Age derives its name from and is roughly analogous with Mythic Europe, the setting of Ars Magica, and is occasionally referred to by that name in older Mage sourcebooks. In Dark Ages: Mage, the Mythic Age overlaps with the Dark Medieval era that spans the entirety of the Dark Ages line of games. By the time of the Dark Fantastic setting of Mage: The Sorcerers Crusade, the Mythic Age has ended, although the wounds of the conflicts that ended it remain fresh to many mages on both sides of the still-nascent Ascension War.