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.