HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>calculateAge</title>
</head>
<body>
  <h1>Age from birth date</h1>
  <pre id="out"></pre>
  <script>
    function calculateAge(birthDate) {
      var today = new Date();
      var age = today.getFullYear() - birthDate.getFullYear();
      var birthdayPassed =
        today.getMonth() > birthDate.getMonth() ||
        (today.getMonth() === birthDate.getMonth() &&
          today.getDate() >= birthDate.getDate());
      if (!birthdayPassed) {
        age -= 1;
      }
      return age;
    }

    var birthDate = new Date(1990, 4, 15);
    document.getElementById("out").textContent =
      "Age: " + calculateAge(birthDate) + " years";
  </script>
</body>
</html>

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JavaScript Date getFullYear() — age calculation (Try It)