The population-level case-fatality rate (CFR) associated with COVID-19 varies substantially, both across countries at any given time and within countries over time . We analyze the contribution of two key determinants of the variation in the observed CFR: the age-structure of diagnosed infection cases and age-specific case-fatality rates . We use data on diagnosed COVID-19 cases and death counts attributable to COVID-19 by age for China, Germany, Italy, South Korea, Spain, the United States, and New York City . We calculate the CFR for each population at the latest data point and also for Italy, Germany, Spain, and New York City over time . We use demographic decomposition to break the difference between CFRs into unique contributions arising from the age-structure of confirmed cases and the age-specific case-fatality . In late June 2020, CFRs varied from 2.2% in South Korea to 14.0% in Italy . The age-structure of detected cases often explains more than two-thirds of cross-country variation in the CFR . In Italy, the CFR increased from 4.2% to 14.0% between March 9 and June 30 , 2020, and more than 90% of the change was due to increasing age-specific case-fatality rates . The importance of the age-structure of confirmed cases likely reflects several factors, including different testing regimes and differences in transmission trajectories; while increasing age-specific case-fatality rates in Italy could indicate other factors, such as the worsening health outcomes of those infected with COVID-19 . Our findings lend support to recommendations for data to be disaggregated by age, and potentially other variables, to facilitate a better understanding of population-level differences in CFRs . They also show the need for well-designed seroprevalence studies to ascertain the extent to which differences in testing regimes drive differences in the age-structure of detected cases.