Extjs Grid with data from json object with sorting

I am trying to display a grid. In this sorting is not working. Here is my code.

    <div id="grid-sample"></div>
    <script type="text/javascript">
        Ext.create('Ext.data.Store', {
            storeId:'cstore',
            fields:['user', 'age', 'place'],
            data: [
                        {"user":"joe","age":27,"place":"sydney"},
                        {"user":"abel","age":29,"place":"delhi"},
                        {"user":"fin","age":18,"place":"san jose"}
                    ]

        });

        Ext.create('Ext.grid.Panel', {
            title: 'Sample grid',
            store: Ext.data.StoreManager.lookup('cstore'),
            autoCreateViewPort: false,
            layout: 'fit',
            columns: [
                { text: 'Name', xtype: 'templatecolumn', tpl: '{user}' ,sortable : true },
                { text: 'Age', xtype: 'templatecolumn', tpl: '{age}' ,sortable : true },
                { text: 'Place', xtype: 'templatecolumn', tpl: '{place}',sortable : true  }
            ],
            width: 750,
            renderTo: 'grid-sample'
        });
    </script>