Django: the value of list_display[4] refers to 'age', which is not a callable, etc

You can't use admin.site.register(User, PersonAdmin), since User and Person are not the same model. Also, it looks like you are trying to include these Person model fields inside the User admin:

from django.contrib.auth.admin import UserAdmin

class PersonInline(admin.StackedInline):
    """ Details a person in line. """
    model = Person
    can_delete = False
    verbose_name_plural = 'person'

    fields = ('username', 'email', 'first_name', 'last_name', 'age', 'city', 'state')

class UserAdmin(UserAdmin):
    inlines = [
        PersonInline
    ]

# Re-register UserAdmin
admin.site.unregister(User)
admin.site.register(User, UserAdmin)