allow age calculation on NULL birthdate and allow age to be null

+ improve translation of age

src/Bundle/ChillPersonBundle/Tests/Controller/PersonControllerUpdateTest.php

@@ -102,7 +102,7 @@ class PersonControllerUpdateTest extends WebTestCase
     public function testHiddenFielsArePresent()
     {
         $crawler = $this->client->request('GET', $this->editUrl);
-
+var_dump($crawler->text());
         $configurables = array('placeOfBirth', 'phonenumber', 'email',
             'countryOfBirth', 'nationality', 'spokenLanguages', 'maritalStatus');
         $form = $crawler->selectButton('Enregistrer')->form(); //;