13. All the characteristics of mental retardation can appear long after birth because of infury to the brain anytime during the period of development upto 18 years of age. injury to the brain can be caused by the following factors:

* Severe malnutrition in the child especially during birth to 2 years of life
* repeated episodes of epileptic fits
* injury to brain from accidents of falls
* strangutation, smoke inhalation , or near drowing
* accidental poisioning