Let's get it out of the way right now.  According to Dr. Mark Borchert, an L.A. based ophthalmologist that was named as one of the best doctor's in Southern California in 2009, there's no evidence to indicate that watching 3D movies will harm you in any way.  "It's not likely to cause any permanent harm to vision," he says.

The only unknown here is children under the age of four.  Binocular vision, as well as stereoscopic vision, is still being worked out at that point.  It's hard to say what the effects of 3D might be on still developing eyes.  On the other hand, without fully developed vision, young kids won't see the 3D effect in the first place.

The good doctor explains that there aren't enough studies on the effect to tell if it could harm children under three or not.  Everyone else though, is fine.  "I can't imagine how this is going to cause any kind of permanent harm to someone who is over four years of age."