Past research has revealed that people often hold overestimated perceptions of their performance in various domains. While there is a debate over how overestimation might facilitate motivation to a certain degree, intentional learning processes typically benefit from accurate judgments. With the focus on lifelong learning, accurate performance judgments should be important in all age groups. However, research on older age groups who do not participate in formal schooling (e.g., middle-aged adults) has primarily investigated performance judgments using laboratory tasks that cannot be equated with complex academic skills. Therefore, the current study investigated age-related differences in global performance judgments (postdictions) and their inaccuracy (or: bias) in adulthood using cross-sectional data from a large-scale, representative German panel study (adults aged 25–67). We investigated judgments of mathematical and reading competence using parametric and nonparametric models to capture the patterns in the data, controlling for educational attainment. In both mathematical and reading competence, we found that while performance judgments decreased, the observed bias increased in older age groups. In addition, there was evidence for small gender differences in performance judgment but not in the bias. The findings provide a comprehensive insight into age-related differences in performance judgments and their accuracy in adulthood.