This study contributes to the literature by examining the evolution of socio-economic disparities in literacy skills between age 15 and 27. It uses combined cross-sectional data from the Programme for International Student Assessment and the Programme for the International Assessment of Adult Competencies in 20 countries and adopts a synthetic cohort approach. In the article we examine if there are differences in the evolution of socio-economic disparities in literacy between the teenage years and young adulthood among high-achieving students (90th percentile) and among low-achieving students (10th percentile) and compare differences across education systems. We used parental education as an indicator of socio-economic status. Findings indicate that on average, disparities in literacy skills observed at age 15 widen by 15% of a standard deviation by age 27. Such increase is determined by a more pronounced growth in achievement among individuals with tertiary educated parents than among individuals whose parents did not complete tertiary education. The increase in socio-economic gaps between age 15 and 27 is more pronounced among low-achieving students. The group that improves the most between age 15 and 27 is low-achieving individuals with tertiary educated parents. We find that socio-economic differences in educational attainment, NEET status and use of skills at home and in the workplace explain a large part of socio-economic disparities in literacy achievement among young adults. Although countries with above-average disparities at age 15 tend to also display above-average disparities at age 27, we identify differences across countries.