A la limite faire un

 
var_dump( $form->getData()); 
$person->setPersBirth(new DateTime("2014-09-25"));
die("fin");
$person = $this->getDoctrine()->getRepository('IfrassMainBundle:Person')->find($form->get('persid')->getData());
$person
->setPersName($form->get('pers_name')->getData())
->setPersFirstName($form->get('pers_firstname')->getData())
->setPersMobil($form->get('pers_mobil')->getData())
->setPersPhone($form->get('pers_phone')->getData())
->setPersEmail($form->get('pers_email')->getData())
->setPersBirth($form->get('pers_birth')->getData());
 

tu reçois bien la date malheureusement cette date est de type string, or doctrine attends un objet de type DateTime :

$person = $this->getDoctrine()->getRepository('IfrassMainBundle:Person')->find($form->get('persid')->getData());
$date = new \DateTime($form->get('pers_birth')->getData() );
$person
->setPersName($form->get('pers_name')->getData())
->setPersFirstName($form->get('pers_firstname')->getData())
->setPersMobil($form->get('pers_mobil')->getData())
->setPersPhone($form->get('pers_phone')->getData())
->setPersEmail($form->get('pers_email')->getData())
->setPersBirth( $date );

Alors dans ces conditions :
http://php.net/manual/fr/datetime.createfromformat.php