Výpočet věku z data narození PHP

public function age($birthdate)
{
    $old = false;
    if ($birthdate < 0)
    {
        $birthdate = abs($birthdate);
        $old = true;
    }

    $birth = DateTime::createFromFormat('U', $birthdate);

    $age = '';
    if ($birth->format('m') == date('m'))
        $age .= 'okolo ';

    if (!$old)
        $age .= $birth->diff(new DateTime('now'))->y;
    else
    {
        $epoch = new DateTime('1970-01-01');
        $diff = $epoch->diff(new DateTime('now'))->y;
        $age .= $diff + $epoch->diff($birth)->y;
    }

    return $age;
}