title: Django: the value of list_display[4] refers to 'age', which is not a callable, etc
body: <p>I'm trying to extend the User model to add additional attributes to my "Person" model when I create instances of it, and I keep getting the following errors:</p>
  
  <pre><code>ERRORS:
  &lt;class 'polls.admin.PersonAdmin'&gt;: (admin.E108) The value of 'list_display[4]' refers to 'age', which is not a callable, an attribute of 'PersonAdmin', or an attribute or method on 'auth.User'.
  &lt;class 'polls.admin.PersonAdmin'&gt;:(admin.E108) The value of 'list_display[5]' refers to 'city', which is not a callable, an attribute of 'PersonAdmin', or an attribute or method on 'auth.User'.
  &lt;class 'polls.admin.PersonAdmin'&gt;: (admin.E108) The value of 'list_display[6]' refers to 'state', which is not a callable, an attribute of 'PersonAdmin', or an attribute or method on 'auth.User'.
  </code></pre>
  
  <p>Person model:</p>
  
  <pre><code>class Person(models.Model):
      """ The model which defines a user of the application. Contains important
      information like name, email, city/state, age, etc """
      user = models.OneToOneField(User, on_delete=models.CASCADE)
      first_name = models.CharField(max_length=200, null=True)
      last_name = models.CharField(max_length=200, null=True)
      email = models.CharField(max_length=200, null=True)
      city = models.CharField(max_length=200, null=True)
      state = models.CharField(max_length=200, null=True)
      age = models.CharField(max_length=50, null=True)
  </code></pre>
  
  <p>Create_account view:</p>
  
  <pre><code>def create_account(request):
      # function to allow a user to create their own account
      if request.method == 'POST':
          # sets a new user and gives them a username
          new_user = User(username = request.POST["username"],
                      email=request.POST["email"],
                      first_name=request.POST["first_name"],
                      last_name=request.POST["last_name"],
                      age=request.POST["age"],
                      city=request.POST["city"],
                      state=request.POST["state"]
                      )
  
          # sets an encrypted password
          new_user.set_password(request.POST["password"])
          new_user.save()
          # adds the new user to the database
          Person.objects.create(user=new_user,
                            first_name=str(request.POST.get("first_name")),
                            last_name=str(request.POST.get("last_name")),
                            email=str(request.POST.get("email")),
                            age=str(request.POST.get("age")),
                            city=str(request.POST.get("city")),
                            state=str(request.POST.get("state"))
                            )
          new_user.is_active = True
          new_user.save()
          return redirect('../')
      else:
          return render(request, 'polls/create_account.html')
  </code></pre>
  
  <p>Snippets of admin.py:</p>
  
  <pre><code>class PersonInline(admin.StackedInline):
      """ Details a person in line. """
      model = Person
      can_delete = False
      verbose_name_plural = 'person'
  
  class PersonAdmin(admin.ModelAdmin):
      """ Defines a Person's information on the admin site
      with displays listed below. """
      list_display = ('username', 'email', 'first_name', 'last_name', 'age', 'city', 'state')
      inlines = (PersonInline, )
  </code></pre>
  
  <p>Any ideas on how to get this working? </p>