function amzius($diena,$menuo,$metai){
return checkdate($menuo,$diena,$metai) ? (((date("m")-$menuo) < 0) || ((date("d")-$diena == 0) && (date("d")-$diena < 0)) ? date("Y")-$metai-1 : date("Y")-$metai): 1;
}

$user_birthdate = explode("-", $data['user_birthdate']);
$amzius = amzius(number_format($user_birthdate['2']),number_format($user_birthdate['1']),$user_birthdate['0']);