Hi Kasai,

You can calculate age using the following script:-

            start = datetime.strptime(object.birthdate, DEFAULT_SERVER_DATE_FORMAT)
            end = datetime.strptime(time.strftime(DEFAULT_SERVER_DATE_FORMAT), DEFAULT_SERVER_DATE_FORMAT)
            delta = end - start
            age = (delta.days / 365)

Please don't forget to import following:-

import time
from dateutil.relativedelta import relativedelta
from datetime import datetime
from openerp.tools import DEFAULT_SERVER_DATE_FORMAT, DEFAULT_SERVER_DATETIME_FORMAT

Hope this helps,, please let me know if it does.