Obi Wan had barely graduated as a padawaan when he got Anakin. But I guess that was a bit of an exception. By the films, I'm not sure theres even any sort of "rules" for when a person can get an apprentice except that it has to be approved by the Jedi Council--but, given the way the Jedi Council views Anakin (not very well) there better be a hell of an explanation as to how they thought it was a good idea to give him one.