Try changing:
$(function(){
$('#dob').datepicker({
changeMonth: true,
changeYear: true,
onClose: closeDatePicker_dob
});
});
to:
var maxBirthdayDate = new Date();
maxBirthdayDate.setFullYear( maxBirthdayDate.getFullYear() - 18,11,31);
$(function(){
$('#dob').datepicker({
changeMonth: true,
changeYear: true,
onClose: closeDatePicker_dob,
maxDate: maxBirthdayDate,
yearRange: '1920:'+maxBirthdayDate.getFullYear()
});
});