It can pose risk to younger children, but once nerves are fully developed there's really no risk. 3D works by basically tricking our eyes with two images in order to simulate depth, and in theory I suppose it could change the way your occipital lobe develops to process the information. I'll use my own eye condition as an example; I have something called Refractive Amblyopia. I was born with a slightly defective lense in my left eye, resulting in blurry vision. My right eye, on the other hand, was/is in perfect condition. So, my brain pretty well adapted to give the right eye dominance while "deactivating" the left. At this point, my left eye is nearly useless, but my right eye has near perfect perception. This condition is reversible up until around the age of eight, so it seems appropriate that most warnings regarding 3D hazards point out children below the age of seven. From my own speculation, I'd guess that a simulation of images in that manner could have a similar effect on the brain, so it would adapt to the simulated 3D environment. If this change wasn't caught and corrected in those early years, it would become a permanent condition. Once again, this is just my own speculation drawn from my own experiences, and a more official scientific opinion should overshadow my own.