Which condition involves symptoms starting as early as infancy and typically around 3 years old?

The condition that involves symptoms starting as early as infancy and typically around 3 years old is pervasive developmental disorder, which encompasses a range of developmental issues, including autism spectrum disorders. Children diagnosed with these disorders may show early signs such as difficulties in communication, social interactions, and patterns of behavior as early as infancy. By the time they reach about 3 years old, these symptoms often become more evident, making the diagnosis clearer.

In contrast, the other conditions listed have different age ranges and symptom onset characteristics. Cystic fibrosis, for example, usually presents with symptoms related to lung and digestive function shortly after birth, while muscular dystrophy often manifests in early childhood but not typically at the same initial developmental stages as pervasive developmental disorders. Cerebral palsy can present at any age but is commonly identified early due to observable mobility difficulties. Therefore, the distinction lies in the nature and timeline of the symptoms associated with each disorder.