Set the yearRange
option. Add yearRange: "1980:2014",
(or whatever start and end years you need):
$("#txtbox").datepicker({
prevText: "",
nextText: "",
changeMonth: true,
changeYear: true,
showButtonPanel: true,
dateFormat: 'mm/yy',
yearRange: "1980:2014",
onClose: function (dateText, inst) {
var month = $("#ui-datepicker-div .ui-datepicker-month :selected").val();
var year = $("#ui-datepicker-div .ui-datepicker-year :selected").val();
$(this).datepicker('setDate', new Date(year, month, 1));
}
});