code: def age(self):
          
          if self.date_range is None:
              return
          dob = self.date_range.middle
          today = datetime.date.today()
          if (today.month, today.day) < (dob.month, dob.day):
              return today.year - dob.year - 1
          else:
              return today.year - dob.year
docs: int, the estimated age of the person.
          
          Note that A DOB object is based on a date-range and the exact date is 
          usually unknown so for age calculation the the middle of the range is 
          assumed to be the real date-of-birth.