JSFiddle - React, Tailwind, and code Playground

HTML

<p>
    <label>how old are you?</label>
     <form id="form" onsubmit="return false;">
      <input   style=position:absolute;top:80%;left:5%;width:40%; type="number" id="userInput">
      <input    style=position:absolute;top:50%;left:5%;width:40%; type="submit"    onclick="age()">
     </form>
 </p>

JavaScript

function age()
      {
        var age = document.getElementById("userInput").value;
        alert(age);
      
      if(age<20) { alert("YOU ARE UNDER 20"); }
      }