JSFiddle - React, Tailwind, and code Playground

HTML

Date of Birth (MM/DD/YYYY): 
 <input type="text" class="datepicker minimumSize" name="BirthDate" id="lastReporteddate"/>
<input type="submit" id="button">

JavaScript

var d = new Date();
var year = d.getFullYear() - 18;
d.setFullYear(year);

$("#lastReporteddate").datepicker({ dateFormat: "dd/mm/yy",
		    changeMonth: true,
		    changeYear: true,
		    maxDate: year,
		    minDate: "-100Y",
            yearRange: '-100:' + year + '',
            defaultDate: d
		 });

$("#button").click(function(){
        var dob = $("#lastReporteddate").val();
        var now = new Date();
        var birthdate = dob.split("/");
        var born = new Date(birthdate[2], birthdate[1]-1, birthdate[0]);
        age=get_age(born,now);
     
        console.log(birthdate[2]+" : "+birthdate[1]+" : "+birthdate[0]);
        console.log(age);
    
        if (age<=18)
        {
       alert("Input Error - Age should be greater then or equal to 18");
            return false;
        }else{
        
        return true;
        
        }
});


    function get_age(born, now) {
      var birthday = new Date(now.getFullYear(), born.getMonth(), born.getDate());
      if (now >= birthday) 
        return now.getFullYear() - born.getFullYear();
      else
        return now.getFullYear() - born.getFullYear() - 1;
    }