Aim: To evaluate treatment-related toxicity, treatment compliance, surgical complications and event-free survival (EFS) in older (≥70 years) versus younger (<70 years) adults who underwent perioperative treatment for gastric cancer. Methods: In the CRITICS trial, 788 patients with resectable gastric cancer were randomised before start of any treatment and received preoperative chemotherapy (3 cycles of epirubicin, cisplatin or oxaliplatin and capecitabine), followed by surgery, followed by either postoperative chemotherapy or chemoradiotherapy (45Gy + cisplatin + capecitabine). Results: 172 (22%) patients were older adults. During preoperative chemotherapy, 131 (77%) older adults versus 380 (62%) younger adults experienced severe toxicity (p < 0.001); older adults received significantly lower relative dose intensities (RDIs) for all chemotherapeutic drugs. Equal proportions of older versus younger adults underwent curative surgery: 137 (80%) versus 499 (81%), with comparable postoperative complications and postoperative mortality. Postoperative therapy after curative surgery started in 87 (64%) older adults versus 391 (78%) younger adults (p < 0.001). Incidence of severe toxicity during postoperative chemotherapy was 22 (54%) in older adults versus 113 (59%) in younger adults (p = 0.541); older adults received significantly lower RDIs for all chemotherapeutic drugs. Severe toxicity rates for postoperative chemoradiotherapy were 22 (48%) older adults versus 89 (45%) for younger adults (p = 0.703), with comparable chemotherapy RDIs and radiotherapy dose. Two-year EFS was 53% for older adults versus 51% for younger adults. Conclusion: Perioperative treatment compliance, especially in the postoperative phase, was poorer in older adults compared with younger adults. As comparable proportions of patients underwent curative surgery, future studies should focus on neo-adjuvant treatment. Trial registration: ClinicalTrials.gov identifier: NCT00407186. EudraCT number: 2006–00413032.