Background: Some behavioral features of Prader-Willi syndrome (PWS) are associated with the major genetic subtypes of this disorder. While most agree that those with maternal uniparental disomy (UPD) have a distinctive cognitive and psychiatric profile, findings are more controversial regarding possible differences among persons who vary in paternal deletion size. Methods: Caregivers of 88 persons with PWS aged 5 to 51 years (M = 22 years) were administered measures of problem behavior, compulsivity, hyperphagia, and adaptive skills. The sample was well characterized as having relatively large, Type I (n = 26) or smaller, Type II (n = 29) deletions, or UPD (n = 33). Results: No significant behavioral differences were found between the Type I versus Type II deletion groups. Within each genetic subtype, however, differences emerged in how advancing age related to behavior. Although age did not emerge as a significant correlate of behavior in the Type II or UPD groups, in the Type I group age was consistently associated with lower problem behaviors, adaptive skills, and externalizing symptoms. Conclusion: Although differences between deletion subtypes were not found, significant within-subtype differences emerged in relationships between age and behavior. Negative associations between age and behavior in the Type I group only may relate to non-imprinted genes that are deleted in Type I but not Type II cases, including CYFIP1. Altered expression of CYFIP1 is seen in other developmental disabilities, including 15q disorders, and haploinsufficiency of CYFIP1 in Type I PWS cases may be associated with age-related phenotypic effects. Findings underscore the importance of a life-span perspective in phenotypic research.