Objective Based on data from the US SEER database, this study investigates the incidence, epidemiological characteristics, treatment modalities, prognosis assessment, and risk stratification of advanced-stage lung adenocarcinoma in young patients. Methods SEER*Stat software was used to collect the incidence rates of lung adenocarcinoma among people under 50 in the US from 2000 to 2020, with annual percentage changes (APCs) calculated using Joinpoint software. Clinical data of 4490 advanced-stage lung adenocarcinoma patients under 50 years old from the SEER database (2010-2018) were retrospectively collected and analyzed. Kaplan-Meier method was used to calculate overall survival, log-rank test to estimate survival rates, and Cox proportional hazards regression model to conduct univariate and multivariate prognostic analyses, and a nomogram model was established to predict the survival of young advanced lung adenocarcinoma patients. The predictive performance of the nomogram was evaluated using ROC curves and calibration curves. X-Tile software was used for risk stratification of the prognosis of young advanced lung adenocarcinoma patients. Results From 2000 to 2020, the incidence rates of young lung adenocarcinoma in men and women in the US decreased from 2.1/100 000 and 2.2/100 000 to 1.1/100 000 and 1.5/100 000 respectively, with APCs of -2.16 and -1.39 (P<0.05). Compared to patients aged 40 to <50 years, patients under 40 were more likely to develop bone metastasis, liver metastasis, and lung metastasis, had a higher probability of receiving chemotherapy, tumors were more likely to occur in the lower or middle lobes of the lung, but had a lower probability of receiving radiotherapy, with statistically significant differences (P<0.05). Cox regression analysis showed that gender, age, race, N stage, M stage, subsite, liver metastasis, and bone metastasis were prognostic factors for young advanced lung adenocarcinoma patients (P<0.05); male patients had worse prognosis than female patients (HR=1.17, 95%CI 1.09-1.25); African American patients had worse prognosis than Caucasian patients (HR=1.12, 95%CI 1.02-1.23), other races had better prognosis than Caucasian patients (HR=0.83, 95%CI 0.77-0.97); patients aged 40 to <50 had worse prognosis than those under 40 (HR=1.34, 95%CI 1.21-1.48); patients with bone metastasis (HR=1.29, 95%CI 1.19-1.40) and liver metastasis (HR=1.40, 95%CI 1.28-1.54) had worse prognosis, with statistically significant differences (P<0.05). Based on the above prognostic factors, a nomogram model was established to predict 1-year, 3-year, and 5-year survival with areas under the curve (AUC) of 0.717, 0.692, and 0.699 respectively, and the calibration curve was close to the 45° diagonal. According to the risk scores, patients were divided into low-risk group [1017 cases (22.7%)], medium-risk group [2871 cases (63.9%)], and high-risk group [602 cases (13.4%)]. The difference is statistically significant comparing the survival rates among three groups (P<0.0001). Conclusion Gender, age, race, N stage, M stage, subsite, liver metastasis, and bone metastasis are prognostic factors for young advanced lung adenocarcinoma patients, and the nomogram model established based on these factors has high predictive performance.