How to disable webform select box with jQuery without resetting selection value to default
You can do this by removing the 'selected' attribute first, and setting the new selected attribute, prior to disabling.

e.g: 

if (age &lt; 19) {
    $("#edit-submitted-bring-alcohol option:selected").removeAttr('selected');
    $("#edit-submitted-bring-alcohol option[value='no']").attr('selected', 'selected')
    $('#edit-submitted-bring-alcohol').val('no');
    $('#edit-submitted-bring-alcohol').prop('disabled', true);
}


Here's a jsfiddle link for tinkering: http://jsfiddle.net/AwUwt/

Edit: Alternatively, you can just hide the select box

if (age &lt; 19) {
    $("#edit-submitted-bring-alcohol option:selected").removeAttr('selected');
    $("#edit-submitted-bring-alcohol option[value='no']").attr('selected', 'selected')
    $('#edit-submitted-bring-alcohol').val('no');
    $('#edit-submitted-bring-alcohol').css('display', 'none');
}