According to the report cited in #2162, the battery in question has a manufacturing date of May 9, 2013. And a failure approximately 7 months later.
We can assume that the fleet-wide age of all 787 batteries was effectively reset to zero on or around May, 2013. So it will be interesting to see if the failure rate vs battery age has improved over the original design or not.
It is noteworthy that the resulting damage in the January 14, 2014 incident appears to be confined to a single cell. So at least the cell spacing and battery box are working as intended.