OOC
That's a good point. Time flows would be different.
I picture it as something like this. Corwin makes the new Pattern right near the end of the Patternfall War, meets Merlin for the first time and tells him his story. At some point thereafter he gets grabbed by Dara and locked away. Avalon was recreated from Corwin's heart at the same time he made the new Pattern, and ruled by a council of the Knights of the Round Table, all of whom believed that Corwin was their rightful king who would return one day.
Corwin escapes Dara's trap when the ghost created by his Pattern switches places with the real deal. The pattern ghost then dissipates after Corwin has had time enough to escape to his universe. Corwin comes to Avalon, discovers that it's the first Shadow of his new reality, and claims his throne as King. That's Year 0. Because time in Chaos and Avalon are different, years passed for Corwin while imprisoned while time passed in Avalon at a different rate.
Age of majority in Avalon is 16, since life expectancy is a lot shorter in a medieval realm. Major life events are marriage, apprenticeship, birth of a child, death of a loved one, and death.