We systematically assessed the sex ratios of mortality rates in the early life course, from birth to age 24 years, for 200 countries and territories from 1990 to 2021. We compiled three extensive databases, including publicly available data sources for children (aged 0–4 years), adolescents (5–14 years), and young people (15–24 years). We modelled the relationship between the sex ratio and total mortality rate for each age group. This study provides insight into levels and trends in sex ratios of mortality and pinpoints countries with outlying sex ratios. We showed that chances of survival up to age 24 years tend to improve more rapidly for girls than boys as total mortality decreases, with a reversal of this trend at very low mortality. Further research should focus on explaining differences across countries and regions, and shed light on the contribution of cause-specific mortality to these sex differences. Targeted interventions or legislation should be taken to reduce sex disparities due to discrimination or excessive exposure to violence.