title: Django QuerySet/Manager filter employees by age from birthday
body: <p>I am a beginner. I would like to get list employees have age > 25 and calculate the average age of employees. Currently, we need to get age from birthday (I have no idea about it ^^)
  . Please help me create sample QuerySet/Manager about it.</p>
  
  <p>Thanks all :))</p>
  
  <p>My model:</p>
  
  <pre><code>class Employee(models.Model):
  """"Employee model"""
  
      first_name = models.CharField(max_length=30)
      last_name = models.CharField(max_length=30)
      birthday = models.DateField(max_length=8)
  </code></pre>
  
  <p>Note: Not get all employees and filter from the results. (<code>results = Employee.objects.all()</code>) and not use Raw SQL.</p>