Edit in JSFiddle

var empData = [{
    'emp_no': '1001',
    'last_name': 'Doe',
        'first_name': 'John',
        'age': '23',
        'doj': '08/06/2009'
        
}, {
    'emp_no': '1002',
    'last_name': 'Sparrow',
        'first_name': 'Jack',
        'age': '26',
        'doj': '08/12/2010'
}, {
    'emp_no': '1003',
    'last_name': 'Spacey',
        'first_name': 'Kevin',
        'age': '27',
        'doj': '01/23/2011'
        
}, {
    'emp_no': '1004',
    'last_name': 'Gates',
        'first_name': 'Bill',
        'age': '35',
        'doj': '10/10/2006'
}];

var empStore = new Ext.data.JsonStore({
    fields: [{
        name: 'emp_no'
    }, {
        name: 'last_name'
    }, {
        name: 'first_name'
    }, {
        name: 'age',
        type: 'int'
    }, {
        name: 'doj',
        type: 'date',
        dateFormat: 'm/d/Y'
    }],
    data: empData
});

function showEmpDetails(val, meta, rec, rIndex, cIndex, store) {
    return "<a href='#'>" + val + "</a>";
    //or apply any css class of your choice using
    //meta.css = ‘awesome-font-style’
    //return val;
}

var grid = new Ext.grid.GridPanel({
    store: empStore,
    columns: [{
        header: 'Emp No.',
        width: 75,
        sortable: true,
        dataIndex: 'emp_no',
        renderer: showEmpDetails
    }, {
        header: 'Last Name ',
        width: 100,
        sortable: true,
        dataIndex: 'last_name'
    }, {
        header: 'First Name ',
        width: 100,
        sortable: true,
        dataIndex: 'first_name'
    }, {
        header: 'Age ',
        width: 75,
        sortable: true,
        dataIndex: 'age'
    }, {
        header: 'Date of Joining ',
        width: 100,
        sortable: true,
        renderer: Ext.util.Format.dateRenderer('m/d/Y'),
        dataIndex: 'doj'
    }],
    viewConfig: {
		//Return CSS class to apply to rows depending upon data values
		getRowClass: function(record, index) {
			var c = record.get('age');
			if (c >= 27) {
				return 'senior';
			} 
		}
	},
    stripeRows: true,
    height: 300,
    width: 440,
    title: 'Employee Details ',
    renderTo: Ext.getBody()
});