Background: Universally, smoking cessation rates among established smokers are poor. Preventing young people from starting use of and becoming addicted to tobacco products remains a key strategy to end the tobacco epidemic. Previous country-specific studies have found that initiation of smoking tobacco use occurs predominantly among young people and have found mixed progress in reducing the prevalence of smoking tobacco use among young people. Current and comparable estimates for all countries are needed to inform targeted interventions and policies.

Findings: Globally in 2019, an estimated 155 million (95% uncertainty interval 150–160) individuals aged 15–24 years were tobacco smokers, with a prevalence of 20·1% (19·4–20·8) among males and 4·95% (4·64–5·29) among females. We estimated that 82·6% (82·1–83·1) of current smokers initiated between ages 14 and 25 years, and that 18·5% (17·7–19·3) of smokers began smoking regularly by age 15 years. Although some countries have made substantial progress in reducing the prevalence of smoking tobacco use among young people, prevalence in 2019 still exceeds 20% among males aged 15–24 years in 120 countries and among females aged 15–24 years in 43 countries.

Interpretation: The fact that most smokers start smoking regularly before age 20 years highlights the unique window of opportunity to target prevention efforts among young people and save millions of lives and avert health-care costs in the future. Countries can substantially improve the health of their populations by implementing and enforcing evidence-based tobacco control policies that prevent the next generation from initiating smoking.