Our goal in the current study was to further investigate the late neurodevelopmental hypothesis of schizophrenia by examining cross-sectional, age-related changes in cognitive function among young adult (a) siblings of individuals with schizophrenia (n = 66), (b) healthy control subjects (n = 77), and (c) the siblings of healthy control subjects (n = 77). All subjects participated in a battery of tasks in four domains: verbal IQ, working memory, episodic memory, and executive function. We found significant group differences in the relationships between age and performance in working memory and episodic memory, with similar patterns for executive function and verbal IQ. The siblings of individuals with schizophrenia showed impaired performance in working memory, episodic memory, and executive function. In addition, healthy control subjects and their siblings showed age-related improvements in all four cognitive domains, whereas the siblings of individuals with schizophrenia showed this result for verbal IQ only.