Adult cognitive age differences in the Wechsler Adult Intelligence Scale-III Canadian normative data were curvilinear for most scales and for the Verbal Comprehension (VC), Perceptual Organization (PO), and Working Memory (WM) factors. These showed stable or increasing scores in early adulthood followed by decreasing scores, necessitating a nonlinear (continuous parameter estimation) approach to modeling adult age differences. Means were as high as or higher than the 18-year-olds until 60 years for PO and 70 years for WM and VC. As the means showed no interpretable differences across 20 to 30 years of adult cohorts, no one year can be identified as the highest scoring. "What If" analyses correcting for possible Flynn effects and for the hypothesis that changes in Processing Speed may underlie some of the age differences suggest that further testing of these two effects is a necessary prerequisite to advancing our understanding of adult age differences. (Contains 3 tables.)