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BACKGROUND: Although the incidence of several cancers diagnosed before age 50 years has increased, contemporary trends in early-onset lung cancer remain poorly defined, particularly across histologic and sociodemographic subgroups.METHODS: A population-based cohort study was conducted via Surveillance, Epidemiology, and End Results 22 Program registries of 81,568 individuals diagnosed with lung cancer at age 20-49 years from 2000 to 2021. Age-adjusted incidence rates and annual percent change (APC) were estimated via National Cancer Institute Joinpoint regression. Overall trends were stratified by histology, sex, race/ethnicity, and state-level smoking prevalence.RESULTS: From 2000 to 2021, early-onset lung cancer incidence declined overall (average APC, -3.81%; 95% CI, -4.01% to -3.64%; p < .0001), which contrasts with rising early-onset colorectal and female breast cancers. Declines among females occurred later but were steeper for lung adenocarcinoma (LUAD; -5.14% vs. -2.82%), whereas males declined faster for non-small cell lung cancer (NSCLC) and overall. Female declines were slower relative to males in Hispanic and non-Hispanic (NH) Black individuals (2.3- and 1.08-fold difference, respectively), whereas NH White females declined 1.3-fold faster than males. Among Asian females, NSCLC incidence did not decline, and LUAD increased through 2017 (+2.45%) and remained stable in Hispanic females. In high-smoking states, male rates declined steadily (-4.32%), whereas female rates were stable until 2010, then dropped sharply, and converged with males by 2021. In low-smoking states, male rates declined 48.75% faster than female rates (-4.76% vs. -3.20%).CONCLUSIONS: Early-onset lung cancer incidence declined overall from 2000 to 2021, except for persistent or rising LUAD among Asian and Hispanic females, which implicates emerging non-tobacco risk factors and highlights the need for targeted prevention.