Background: The alarming rise in the incidence of gastric (GC) and colorectal (CRC) adenocarcinomas in young adults (YA) over the past three decades is not well understood. How YA populations differ from older patients with the same gastrointestinal malignancies warrants further investigation. Methods: We retrospectively analyzed the California Cancer Registry and the Office of Statewide Health Planning and Development data for all GC and CRC cases from 2000 to 2012. Pearson’s Chi-square analysis was used to analyze differences in demographic, clinical and histopathologic features and log-rank test to compare survival between young (≤ 40 years old) and older adults (40-90 years old) with GC or CRC. Results: Of the GC (n = 19,368) and CRC (n = 117,415) patients included in the study, YA accounted for 4.5% (n = 883) of GC and 3.2% (n = 3723) of CRC. Hispanic ethnicity was more common in YA for both cancers compared to older patients (50.9% vs 26.8% GC, 29.6% vs. 15.7% CRC, p < 0.0001). YA were more likely to have poorly differentiated (74.6% vs. 59.8% GC, 22.5% vs 17.5% CRC, p < 0.0001), higher grade (77.0% vs 61.6% GC, 23.9% vs 18.6 CRC, p < 0.0001), and signet ring features (44.6% vs 21.0% GC, 3.2% vs 1.1% CRC, p < 0.0001) compared to older patients. Synchronous peritoneal metastases were more common in YA compared to older patients (32.1% vs. 14.1% GC, 8.8% vs 5.4% CRC, p < 0.0001). YA with GC or CRC had a greater 5-year survival compared with older patients with the same stage of malignancy. Subgroup analysis of Stage I GC demonstrated lower survival in YA compared with adults aged 41-49 and 50-64 years (65.1% vs. 70.7% and 69.1%, 95% CI 49.7-76.9%, 62.5-77.3%, 65.2-72.7% respectively). Conclusions: GC and CRC in young adults have distinctly worse clinical and histopathologic features compared to older patients with the same malignancy. Ethnic disparity exists in the YA patients. This study contributes to improving the understanding of younger versus older GI cancer patients.