code: def age(self, as_at_date=None):
          
          if self.date_of_death != None or self.is_deceased == True:
              return None
  
          as_at_date = date.today() if as_at_date == None else as_at_date
          
          if self.date_of_birth != None:
              if (as_at_date.month >= self.date_of_birth.month) and (as_at_date.day >= self.date_of_birth.day):
                  return (as_at_date.year - self.date_of_birth.year)
              else:
                  return ((as_at_date.year - self.date_of_birth.year) -1)
          else:
              return None
docs: Compute the person's age