Since Django 1.5 you may easily extend the user model and keep a single table on the database.
from django.contrib.auth.models import AbstractUser
from django.db import models
from django.utils.translation import ugettext_lazy as _
class UserProfile(AbstractUser):
age = models.PositiveIntegerField(_("age"))
You must also configure it as current user class in your settings file
# supposing you put it in apps/profiles/models.py
AUTH_USER_MODEL = "profiles.UserProfile"
If you want to add a lot of users' preferences the OneToOneField option may be a better choice thought.
A note for people developing third party libraries: if you need to access the user class remember that people can change it. Use the official helper to get the right class
from django.contrib.auth import get_user_model
User = get_user_model()