Right, the issue is that jqxgrid requires the age attribute to be present on the first record in the source array, which isn’t the case in my scenario.
Here is a fiddle demonstrating the issue in the latest jqwidgets.
http://jsfiddle.net/1ah9ph8g/14/
If you replace the current source with the commented out one (simply a reversal of the records), the age displays properly.