I have to edit code  to openerp/addons/hr/hr.py

import time
from datetime import datetime
from openerp.tools import DEFAULT_SERVER_DATE_FORMAT
...
class hr_employee(osv.osv):
    _name = "hr.employee" 
    _description = "Employee" 
    _order = 'name_related'
    _inherits = {'resource.resource': "resource_id"}
    _inherit = ['mail.thread']

...

    def get_age(self):
        if not  self.birthday:
            return 0
        birth = datetime.strptime(self.birthday, DEFAULT_SERVER_DATE_FORMAT)
        today = datetime.strptime(time.strftime(DEFAULT_SERVER_DATE_FORMAT), DEFAULT_SERVER_DATE_FORMAT)
        delta = today - birth
        return delta.days / 365