Malignancies of the upper gastrointestinal tract are rare in early-onset patients outside the hereditary genetic disorders. There are few reports describing adenocarcinoma of the esophagogastric junction (AEG) in extremely early-onset patients aged under 50years old. The aim of this study was to describe the clinicopathological features and prognosis of adenocarcinoma of esophagogastric junction (AEG) in early-onset patients among three successive periods: 1975-1989 (period 1), 1990-2004 (period 2), and 2005-2017 (period 3). Between 1975 and 2017, data were extracted from the Surveillance, Epidemiology, and End Results database, and 18,278 patients with AEG were enrolled. Three age groups of patients were identified: < 50, 50-69, and ≥ 70years of age. Clinicopathological characteristics and prognostic outcomes were reviewed and compared among three groups over three periods (1975-89, 1990-04, and 2005-2017). Multivariate Cox regression analysis was performed to adjust for covariate effects related to both overall survival (OS) and cancer-specific survival (CSS). Among three age groups, early-onset patients were more likely to present with higher tumor grade, advanced nodal, and distant metastatic disease at diagnosis than other groups (p < 0.01 for both). In comparison to the older group, a higher proportion of patients in the early-onset group received chemotherapy and radiation treatment. After adjusting for covariates, early-onset patients had a better CSS and OS than elderly patients. Early-onset AEG patients were more likely than other age groups to present with advanced disease upon diagnosis. However, the prognosis of early-onset patients was better than their older counterparts after adjustment for covariates. The dissimilarities in tolerance to treatment among early-onset, middle-aged, and elderly patients could be the reason for this difference.