There's a warning for the Nintendo 3DS that cautions that children under the age of six may be affected by the system's 3D display. On a Japanese information page at Nintendo, they recommend that children six and under not use the 3DS' 3D effects in their general safety warnings.
Nintendo says that kids' vision is still in the developmental stage, and viewing 3D content of any kind could impact their eyesight.
All is not lost if you're planning on buying a 3DS for your five year old. Nintendo reminds that you can slide the 3D slider all the way down to get a plain ol' 2D image, making it safe for eyeballs of any age.
I wonder how far Nintendo will go to inform consumers on this front. I'd guess the system's splash screen would contain a warning. People love to sue Nintendo, as I'm sure they know by now.