JSFiddle - React, Tailwind, and code Playground
HTML
<select size="1" class="day">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option>
<option value="8">8</option>
<option value="9">9</option>
<option value="10">10</option>
<option value="11">11</option>
<option value="12">12</option>
<option value="13">13</option>
<option value="14">14</option>
<option value="15">15</option>
<option value="16">16</option>
<option value="17">17</option>
<option value="18">18</option>
<option value="19">19</option>
<option value="20">20</option>
<option value="21">21</option>
<option value="22">22</option>
<option value="23">23</option>
<option value="24">24</option>
<option value="25">25</option>
<option value="26">26</option>
<option value="27">27</option>
<option value="28">28</option>
<option value="29">29</option>
<option value="30">30</option>
<option value="31">31</option>
...
JavaScript
$(".year").keyup(function(){
var day = $(".day option:selected").val();
var month = $(".month option:selected").val();
var year = $(".year").val();
var bdate = month + '/' + day + '/' + year;
var fullYear = new Date();
fullYear.getFullYear();
var date = new Date();
date.setTime(Date.parse(bdate));
var year1 = date.getFullYear();
date.setTime(Date.parse(fullYear));
var year2 = date.getFullYear();
var age = year2 - year1;
if (year.length == 4) {
$("#takebdate").html(age);
}
});