The belief that people are generally fair and trustworthy has generated plenty of scholarly attention in recent decades, particularly in the Scandinavian countries, which are often known for high levels of social trust. This article draws attention to the current discussion in the literature on whether social trust is a stable cultural trait marked by persistence or is based on experiences and subject to change throughout life. Based on unique longitudinal data from five different cohorts of young people in Sweden, ranging in age from 13 to 28 years, this article provides an empirical contribution on how social trust develops over time. The results show that there is a greater degree of instability in social trust between 13 and 15 years of age than in other age groups, and that social trust appears to stabilize with age. Findings also indicate that there are substantial inter-individual differences in social trust among young people within the same age group, both in initial levels and in the rates of change over time. The article concludes that although social trust is relatively stable it tends to crystallize in early adulthood, highlighting the relevance of the impressionable-years hypothesis.