<?
function fn_age($yy,$mm,$dd)
{
 $now_year = date("Y");
 $now_month = date("m");
 $now_day = date("d");

  if($mm < $now_month) 
      $age= $now_year - $yy;
  else if($mm == $now_month) {
      if($dd <= $now_day)
        $age= $now_year - $yy;
      else
        $age= $now_year - $yy-1;
  }
  else
      $age= $now_year - $yy-1;

  return $age; 
}

$p_age = fn_age('1899','04','20');
echo $p_age;
?>

자료인용
http://ifull.co.kr/zbxe/5150