employee.py

from django.db import models

class Employee(models.Model):
  name = models.CharField(max_length=50)
  birth_day = models.DateField()
  department = models.ForeignKey('Department')

class Department(models.Model):
  sector = models.CharField(max_length=255)


# query oldest
oldest = []
for dept in Department.objects.all():
  try: # case there is department without employee 
    employee = dept.employee_set.latest('birth_day')
    oldest.append(employee)
  except Employee.DoesNotExist:
    pass

print oldest