Very young in fact -- I think I remember reading an article that claimed that very young babies could "read" facial expressions, at least in the happy-versus-sad sense.<p>I can't find any references to it online in the thirty seconds or so I'm willing to spend on it, but I did find:<p><a href="http://www.boston.com/news/science/articles/2009/11/10/boston_lab_explores_childrens_complex_lessons_in_reading_faces/?page=full" rel="nofollow">http://www.boston.com/news/science/articles/2009/11/10/bosto...</a><p>...which says that babies start learning facial expressions at 6 months of age.<p>Autism is different though; it's not just a lack of ability, but a lack of interest. I distinctly remember reading that, for example, during a movie where the scene is primarily depicting two people talking, a "normal" person will watch the faces of the actors, while the "autistic" subject would tend to focus on the light switch in the background. I doubt this is universal, and the article may have even been about one person in particular, but at this point correlations between autism and difficulty with interpersonal emotional interpretation are accepted as generally true.<p>...I think this has an interesting, though probably coincidental, consequence: sci-fi shows and movies tend to focus much more heavily on objects and technology than faces and acting.