Children with mental retardation often do not exhibit symptoms until preschool age, when they begin to experience delayed language development, motor development, and social skills. They may be slow to dress themselves and eat on their own. Affected children may appear disinterested or may exhibit behavioral problems often linked to frustration. They lag behind their peers in achieving major developmental milestones.
Infants may also exhibit symptoms, though more rarely. Abnormalities at birth may indicate more severe mental retardation. They include physical abnormalities such as an enlarged or unusually small head, unusual facial features and deformed hands or feet. Affected infants may show signs of physical illness including seizures, throwing up, and lack of interest.