Background Minimum legal drinking age (MLDA) is an effective policy tool in preventing youth drinking and shortterm alcohol-attributable harm, but studies concerning long-term associations are scarce. Methods In this register-based, national cohort study, we assessed alcohol-attributable morbidity and mortality of cohorts born in 1944-54 in Finland. Data were from the 1970 census, the Care Register for Healthcare (maintained by the Finnish Institute of Health and Welfare), and the Cause-of-Death Register (maintained by Statistics Finland). As MLDA was lowered from 21 years to 18 years in 1969, these cohorts were effectively allowed to buy alcohol from different ages (18-21 years). We used survival analysis to compare their alcohol-attributable mortality and hospitalisations with a 36-year follow-up. Findings Compared with the first cohort (1951) allowed to buy alcohol from age 18, the hazard ratios (HRs) for alcohol-attributable morbidity and mortality were lower in cohorts who could not buy alcohol until age 20 or 21 years. For alcohol-attributable morbidity in those aged 21 years when the reform took place, HR was 0 center dot 89 (95% CI 0 center dot 86-0 center dot 93) for men and 0 center dot 87 (0 center dot 81-0 center dot 94) for women versus those aged 17 years. For alcohol-attributable mortality, HR was 0 center dot 86 (0 center dot 79-0 center dot 93) for men and 0 center dot 78 (0 center dot 66-0 center dot 92) for women aged 21 years when the reform took place. The outcomes of the later-born 1952-54 cohorts did not differ from the 1951 cohort. Interpretation Earlier cohorts had consistently lower alcohol-attributable mortality and morbidity; however, other simultaneous increases in alcohol availability probably contributed to increased alcohol-related harm among the younger cohorts. Overall, differences between cohorts born only a few years apart highlight late adolescence as a crucial period for the establishment of lifelong patterns of alcohol use and suggest that higher MLDA could be protective for health beyond young adulthood.