JSFiddle - React, Tailwind, and code Playground

HTML

<div id="nominee_one_dob"></div>

JavaScript

$(document).ready(function() {
  $( "#nominee_one_dob" ).datepicker({
    changeMonth: true,
    changeYear: true,
    dateFormat: 'yy-mm-dd',

    onSelect: function (date) {
         var dob = new Date(date);
         var today = new Date();
     
        if (dob.getFullYear() + 18 < today.getFullYear())
        {
            alert("Under 18");
        }
        else
        {
            alert(" Over 18");
        }
    }

  });
});