I am not sure that there is an effective innoculation until the child has acquired the intellectual/emotional ability/level/sophistication to comprehend/internalize that you cannot assume the truth/correction of anything you hear/read. While I cannot pinpoint when it occurred, it was obvious to me that DS10 was over that particular hurdle at age 5. Fortunately when he was very young, I don't think that he particularly cared or paid attention to strangers' comments. These days, I don't need to say a word as while DS remains polite, he does get this look (not as rude/obvious as eye-rolling) that confirms that he recognizes the hyperbole.
