Microsoft JScript runtime error: 'WebService' is undefined ?

function InitializeService(){
        service.useService('http://localhost:4656/WebToJson.asmx?op=GetAge');

// this line giving problem can you help me any one 


    }
    var StrYear, StrMonth, StrDay;
    function GetAge(){
     StrYear = document.DemoForm.StringYear.value;
     StrMonth = document.DemoForm.StringMonth.value;
     StrDay = document.DemoForm.StringDay.value;
     service.GetAgeService.callService("GetAge", StrYear, StrMonth, StrDay);
   
    }