Compared with cases diagnosed at age 50 years and older, younger patients had higher odds of presenting with somatic variations in PIK3CA, SMAD3, and TSC2, whereas younger patients had decreased odds of presenting with somatic variations in GNAS. Differences in GNAS by age at onset were also noted in stratified analyses for cases diagnosed with mucinous adenocarcinomas of the appendix. This evidence concerns the gene PIK3CA and mucinous adenocarcinoma of the appendix.