A
myth.
Children can be difficult to deal with at any age. A four year old is just as likely to be upset at something going wrong as a two year old.
So is a sixteen year old, for that matter. The difference is how the child deals with his or her emotions. Any child without proper care and upbringing can learn to be "Terrible".
see also: yossarian. Beware of that picture. *shudder*