This study estimates and decomposes components of different measures of inequality in health and healthcare use among millennial adolescents, a sizeable cohort of individuals at a critical stage of life. Administrative data from the UK Hospital Episode Statistics are linked to Next Steps, a survey collecting information about millennials born between 1989 and 1990, providing a uniquely comprehensive source of health and socioeconomic variables. Socioeconomic inequalities in psychological distress, long-term illness and the use of emergency and outpatient hospital care are measured using a corrected concentration index. Shapley-Shorrocks decomposition techniques are employed to measure the relative contributions of childhood socioeconomic circumstances to adolescents' health and healthcare inequality of opportunity. Results show that income-related deprivation contributes to significant inequalities in mental and physical health among adolescents aged between 15 and 17 years old. There are also pro-rich inequalities in the use of specific outpatient hospital services (e.g., orthodontic and mental healthcare), while pro-poor disparities are found in the use of emergency care services. Regional and parental circumstances are leading factors in influencing inequality of opportunity in the use of hospital care among adolescents. These findings shed light on the main drivers of health inequalities during an important stage of human development and have potentially important implications on human capital formation across the life-cycle.