Objective: Few studies have investigated the neuropsychological impact of aging on developmental disorders, such as autism spectrum disorder (ASD). This lack of information on the long-term neuropsychological trajectory of developmental disorders limits our ability to discern cognitive profiles occurring later in life as due primarily to the expected or typical course of the developmental disorder or to an atypical, pathological process (e.g., dementia). The current case study describes a 56-year-old African-American male with ASD referred for assessment with concerns of dementia-related memory decline. Method: Client was diagnosed with “infantile autism” at age 3, and engages in some independent activities as an adult. Medical history is positive for hypertension, treated with losartan. Family noticed declining cognition (e.g., trouble recalling events, following multi-step instructions) over the past 2 years. ADOS-2 and SRS-2 confirmed ASD diagnosis. Neuropsychological tests measured cognition (IQ, language, memory, visuospatial skills, executive functioning). Informant-report measures assessed adaptive function and dementia-related symptoms among intellectually disabled individuals. Results: Neuropsychological testing revealed average IQ, memory, and visuospatial abilities, with mildly impaired language and executive functioning. Informant-report measures revealed impairment (as expected) but no recent change in adaptive skills and no evidence of dementia-related symptoms. Conclusion: Dementia was not diagnosed because selective impairment in language, executive functioning, and adaptive skills is consistent with both ASD and early Alzheimer's disease; therefore, it is unclear whether this profile represents an expected versus pathological aging process for this individual, and what best explains the reported change in cognitive status. This case highlights the need to better characterize aging in neurodevelopmental populations.