fredboniface.co.uk/php/age.php

<?php

function getAge($birth_year, $birth_month) {
  $cur_year = date("Y");
  $cur_month = date("m");

  if ($birth_month > $cur_month) {
    return (($cur_year - 1) - $birth_year);
  } else {
    return ($cur_year - $birth_year);
  }
}