Final answer:
Motor stereotypies typically start around the age of one to two years old. These repetitive and purposeless movements are common in children with neurodevelopmental disorders.
Step-by-step explanation:
Motor stereotypies typically start around the age of one to two years old. Motor stereotypies are repetitive, rhythmic, and purposeless movements, such as body rocking, hand flapping, or finger flicking. These stereotypies are common in children with neurodevelopmental disorders, such as autism spectrum disorder and intellectual disability.