The youngest of its fans should probably avoid playing in 3D, according to Reggie Fils-Aime.
Speaking to Kotaku, Reggie Fils-Aime has stated that while playing the Nintendo 3DS does not present any health issues, children under the age of seven should probably avoid playing in 3D.

Fils-Aime was quite clear on the matter. "We will recommend that very young children not look at 3D images. That's because, [in] young children, the muscles for the eyes are not fully formed...This is the same messaging that the industry is putting out with 3D movies, so it is a standard protocol. We have the same type of messaging for the Virtual Boy, as an example."
The Nintendo 3DS is due for release by March 2011.