Which developmental condition is characterized by decreased intellectual capacity and typically manifests before age eighteen?

Intellectual disability is a developmental condition marked by significant limitations in both intellectual functioning and adaptive behavior. This typically manifests during the developmental period, which is considered to be before the age of eighteen. The key characteristics of intellectual disability include difficulties in learning, reasoning, problem-solving, and adaptive skills, which are essential for everyday living.

In contrast to other options, such as Down's Syndrome, which is a specific genetic condition that can result in intellectual disabilities but is not synonymous with the term itself, intellectual disability is broader and encompasses a variety of causes and presentations. Expressionism refers to a movement in art and literature, and optimal complexity is a concept related to the balance of challenge and skill in learning or therapeutic contexts, neither of which pertain to developmental conditions or intellectual capacity. Thus, identifying the condition directly associated with decreased intellectual capacity clearly aligns with the definition of intellectual disability.