From an IC perspective as far as shards go I've always gone with this:
1. When the scream happened that linked everyone to their reltos, this signaled the end of Until Uru (and the shards were all instances. So each shard was it's own instance of the city etc.)
2. Then MOUL happened and we were all brought together again.
3. MOUL ended and until my IC self found a shard she was alone in the city again. Then MOULa started, and now all shards are instances of MOULa, thus explaining the differences between all of the various shards. SO Each shard is it's own instance of the City and it's Ages.