The most notable point Nintendo makes is that children under the age of six should not use the 3D effect on a Nintendo 3DS device (however, Nintendo does point out that there is 2D-conversion, so you can still drop the extra cash for a 3DS instead of just buying a regular DS). The reasoning behind this is that the 3DS delivers two different images, one for each eye–and because children’s vision is still developing, viewing 3D could negatively impact your kids’ eyes.
This is not only a warning for the Nintendo 3DS, though–it’s a warning for 3D in general. All commercial 3D (movies, HDTVs, etc.) utilizes the same trick as the Nintendo 3DS, giving viewers a different image for each eye. So, it would logically follow that children under the age of six also should not watch 3D movies or 3D HDTV.
The human eyes’ ability to perceive depth in the real world is called stereopsis, which is the process in visual perception that’s not complete until around six or seven. If stereopsis does not fully develop, children will have a condition called strabismus, or “lazy eye,” in which eyes don’t focus on the same object and depth perception is thus compromised.
Stereoscopic images–aka 3D images–appear to most people to be in 3D because the eyes are being tricked into not focusing on the same object. In other words, when you view images in 3D you are temporarily giving yourself lazy eye.
Along with the “not for kids under six” warning, Nintendo also gives some general notes–because 3D imagery is more likely to give you eye-fatigue, Nintendo suggests a break after every 30 minutes (instead of every hour). Also, Nintendo notes that the 3DS has an included 3D slider for controlling the 3D-ness of the image (but also warns that some people may not be able to see the 3D images at all, no matter how hard they try).