In a new set of guidelines, the World Health Organization said that infants under 1 year old should not be exposed to electronic screens and that children between the ages of 2 and 4 should not have more than one hour of “sedentary screen time” each day. Limiting, and in some cases eliminating, screen time for children under the age of 5 will result in healthier adults, the organization, a United Nations health agency, announced on Wednesday.