Large-scale international assessments, such as PIAAC (Programme for the International Assessment of Adult Competencies) or PISA (Programme for International Student Assessment), have heightened interest in the comparative analysis of factors influencing the formation of skills and their change over time. However, such comparison has been hindered by substantial differences in contexts that can only be partially controlled in statistical analysis (e.g., cultural, social, or geographical factors particular to jurisdictions). In this article, we analyze a "natural experiment," trends in the development of learning outcomes in the Czech and Slovak Republics following the dissolution of the joint state in 1993. The analysis is theoretically based on an age cohort-period framework. We find that in both countries, PISA results are closely related to the future abilities of the particular age cohort (as measured in PIAAC), which supports the thesis that education has a longlasting effect. Although both countries followed similar trajectories, we find that some noticeable trends can be explained by different policies following dissolution of the joint state.