Intellectual disability starts any time before a child turns 18 and is characterized by problems such as:
Intellectual functioning or intelligence: Which includes the abilities to learn, reason, problem solve, and other skills
Adaptive behavior: Which includes everyday social and life skills.
The term “developmental disabilities” is a broader category of disabilities that are often life-long. These disabilities can be intellectual, physical, or both.
“IDD” is a term that is sometimes used to describe situations in which intellectual disability and other disabilities are present.
It might be helpful to think about IDDs in terms of the body parts or systems they affect or how they occur. For example:
Nervous system
These disorders affect how the brain, spinal cord, and nervous system function, which can affect intelligence and learning. These conditions can also cause other problems such as behavioral disorders, speech or language difficulties, seizures, and trouble with movement. Cerebral palsy,5Down syndrome, Fragile X syndrome, and autism spectrum disorders (ASDs) are examples of IDDs related to problems with the nervous system.
Sensory system
These disorders affect the senses (sight, hearing, touch, taste, and smell) or how the brain processes or interprets information from the senses. Preterm infants and infants exposed to infections, such as cytomegalovirus, may have problems with their eyesight and/or hearing. In addition, being touched or held can be difficult for people with ASDs.
Metabolism
These disorders affect how the body uses food and other materials for energy and growth. For example, how the body breaks down food during digestion is a metabolic process. Problems with these processes can upset the balance of materials available for the body to function properly. Too much of one thing, or too little of another can cause problems with overall body and brain function. Phenylketonuria (PKU) and congenital hypothyroidism are examples of metabolic conditions that can lead to IDDs.
Degenerative
Individuals with degenerative disorders may seem or be normal at birth and may develop normally for a time, but then they begin to lose skills, abilities, and functions because of the condition. In some cases, the problem may not be detected until the child is an adolescent or adult and starts to show signs of loss of function. Some degenerative disorders result from other conditions, such as untreated problems of metabolism.

More than 200,000 cases per year are diagnosed in the United States. Down syndrome causes a distinct facial appearance, intellectual disability, developmental delays, and may be associated with thyroid or heart disease. Down Syndrome is a complicated disorder. However, most experience one or more of the following:

Developmental: Delayed development, learning disability, short stature, or speech delay in a child Eyes:  Lazy eye or spots

Also common:  Difficulty thinking and understanding, brachycephaly, upslanting palpebral fissures, atlantoaxial instability, bent little finger, congenital heart disease, displacement of the tongue, excess skin on the back of the neck, flaccid muscles, hearing loss, immune deficiency, low-set ears, mouth breathing, obesity, obstructive sleep apnea, polycythemia, seborrheic dermatitis, single line on palm, thickening of the skin of the palms and soles, thyroid disease, or vision disorder.

While there is no cure for Down Syndrome, Early intervention programs, with a team of therapists and special educators treating each person’s specific situation, are helpful in managing Down syndrome. Treatment consists of therapy as well as self-care.

The term intellectual disability covers the same population of individuals who were diagnosed previously with mental retardation in number, kind, level, type, duration of disability, and the need of people with this disability for individualized services and supports. While intellectual disability is the preferred term, it takes time for language that is used in legislation, regulation, and even for the names of organizations, to change.

“Developmental Disabilities” is an umbrella term that includes intellectual disability but also includes other disabilities that are apparent during childhood. Developmental disabilities are severe chronic disabilities that can be cognitive or physical or both. The disabilities appear before the age of 22 and are likely to be life-long. Some developmental disabilities are largely physical issues, such as cerebral palsy or epilepsy. Some individuals may have a condition that includes a physical and intellectual disability, for example Down syndrome or fetal alcohol syndrome.

There are a number of causes. Our understanding of the causes of Intellectual disabilities focuses on the types of risk factors (biomedical, social, behavioral, and educational) and the timing of exposure (prenatal, perinatal, and postnatal) to those factors.   

Many people diagnosed with Cerebral Palsy will experience some or even all of the following:

Muscular: Difficulty walking, difficulty with bodily movements, muscle rigidity, permanent shortening of muscle, problems with coordination, stiff muscles, overactive reflexes, involuntary movements, muscle weakness, muscle spasms, or paralysis of one side of the body

Developmental: Failure to thrive, learning disability, slow growth, or speech delay in a child

Speech: Speech disorders or stuttering

Also common: Constipation, difficulty swallowing, drooling, hearing loss, leaking of urine, paralysis, physical deformity, scissor gait, seizures, spastic gait, teeth grinding, tremor, or difficulty raising the foot