function checkBirthday(obj)
 {
    if( !/^(\d{1,2})\/(\d{1,2})\/(\d{4})$/.test(obj.value) ) {
        alert( "Invalid date supplied - must be format DD/MM/YYYY" );
        //obj.focus();
        return;
    }

    var d = new Date();
    var d2 = new Date(RegExp.$3, RegExp.$2, RegExp.$1);

    var diff = d.getDiff(d2, "y")
    if( isNaN(diff) ) {
        alert( "Invalid date supplied" );
    }
    else if( diff < 2 ) {
        alert( "This IS an INFANT \n they are younger that 2" );
        infant = true;
        document.form.submit();

    }
    else if( diff > 2 ) {
        alert( "This is NOT an INFANT \n they are older that 2" );
        infant = false;
        document.form.birthday.value = "";
        document.form.date.focus();
    }

}
<img src="g_toolbar_flights.gif" onClick="javascript: checkRetdt(document.form.retdt);" onMouseOver='style.cursor="hand"'>