The literature is mixed about whether age of onset is a useful variable in explaining the heterogeneity of late-life bipolar disorder. The aim of this study was to examine the relationship of age of onset with clinical, family history, and neuropsychological functioning in middle-aged and older patients with bipolar disorder. A total of 87 outpatients with bipolar disorder with a mean age of 59 (range, 42-89) were included in this study. Age of onset was analyzed as a continuous variable and was split at age 40 years into early-onset and late-onset groups. Participants were administered measures of psychopathology, cognitive functioning, and medication usage. Few meaningful differences emerged between early-onset and late-onset groups, except that overall psychopathology was significantly lower in the late-onset group. Age of onset did not relate to differences in family history, depressive symptoms, cognitive functioning, or medication use whether used as a categorical or continuous variable. Thus, the validity of late-onset bipolar disorder as a distinct syndrome was not corroborated by this study. Interpretation of these findings is limited by the sample size, cross-sectional design, and a lack of brain imaging data. Further research on the clinical features and neurobiological aspects of late-life bipolar disorder is needed.