||CONTEXT: For the last 50 years, overall age-standardized incidence rates for noncardia gastric cancer have steadily declined in most populations. However, overall rates are summary measures that may obscure important age-specific trends. OBJECTIVE: To examine effects of age at diagnosis on noncardia gastric cancer incidence trends in the United States. DESIGN, SETTING, AND PARTICIPANTS: Descriptive study with age-period-cohort analysis of cancer registration data from the National Cancer Institute's Surveillance, Epidemiology, and End Results Program, which covers approximately 26% of the US population. From 1977 through 2006, there were 83,225 adults with incident primary gastric cancer, including 39,003 noncardia cases. MAIN OUTCOME MEASURES: Overall and age-specific incidence rates, adjusted for period and cohort effects using age-period-cohort models. Results were stratified by race, sex, and socioeconomic status. RESULTS: Overall age-standardized annual incidence per 100,000 population declined during the study period from 5.9 (95% confidence interval [CI], 5.7-6.1) to 4.0 (95% CI, 3.9-4.1) in whites, from 13.7 (95% CI, 12.5-14.9) to 9.5 (95% CI, 9.1-10.0) in blacks, and from 17.8 (95% CI, 16.1-19.4) to 11.7 (95% CI, 11.2-12.1) in other races. Age-specific trends among whites varied significantly between older and younger age groups (P < .001 for interaction by age): incidence per 100,000 declined significantly from 19.8 (95% CI, 19.0-20.6) to 12.8 (95% CI, 12.5-13.1) for ages 60 to 84 years and from 2.6 (95% CI, 2.4-2.8) to 2.0 (95% CI, 1.9-2.1) for ages 40 to 59 years but increased significantly from 0.27 (95% CI, 0.19-0.35) to 0.45 (95% CI, 0.39-0.50) for ages 25 to 39 years. Conversely, rates for all age groups declined or were stable among blacks and other races. Age-period-cohort analysis confirmed a significant increase in whites among younger cohorts born since 1952 (P < .001). CONCLUSIONS: From 1977 through 2006, the incidence rate for noncardia gastric cancer declined among all race and age groups except for whites aged 25 to 39 years, for whom it increased. Additional surveillance and analytical studies are warranted to identify risk factors that may explain this unfavorable trend.