JSFiddle - React, Tailwind, and code Playground

HTML

<select name="" id="agerange" multiple>
  <option value="1">1</option>
  <option value="2">2</option>
  <option value="18" selected>18</option>
  <option value="33" selected>33</option>
  <option value="45">45</option>
  <option value="67">67</option>
</select>
<div></div>

JavaScript

$( "select" ).change(function() {
  $("#agerange option").css("border", "");
  var i=0;
  $("#agerange option:selected").each(function() {
    i++;
  	var str = "";
    $( "select option:selected" ).each(function() {
      str += $( this ).text() + " ";
    });
    $( "div" ).text( str );
});

var option_val = $('#agerange option:selected').val();
    if (i == 1 && option_val == 18) {
     	$('#agerange option:selected[value=18]').css("border", "1px solid red");
      console.log('agerange3');
    }
    
  });