BackgroundIdentification of reliable risk-stratification tools is critical for surgical decision making, particularly in frail and elderly. The aim of the study is to validate the Emergency Surgery Frailty Index (EmSFI), in over 65 years old patients operated on for acute appendicitis.MethodsAn observational study was conducted enrolling elderly patients with diagnosis of acute appendicitis who underwent emergency appendicectomy or right colectomy, between 2016 and 2021. All patients were treated according to the last SIFIPAC/WSES/SICG/SIMEU guidelines.ResultsOverall, 61 patients were analyzed. Complication rate was higher for patients in the second EmSFI risk Class. Moreover, ROC analyses identified 3 as the best cutoff value in predicting risk of adverse postoperative events. Complication rate was higher in oldest elderly patients—over 80 years—(42.9 vs 22.5%;p0.05) and was mainly related to medical complications (42.9 vs 12.5%,p0.007). However, intestinal obstruction, peri-appendicular abscess on preoperative CT, peritonitis and a longer duration of surgery are related with increased risk of complications in the group of patients under 80 years.ConclusionThe EmSFI score results a valid prognostic marker for frailty status, and it may support the surgeon in emergency setting for acute appendicitis. Patients aged 80 years or older have a higher risk of complications, independent from those factors which relate to increased morbidity in younger elderly patients. Age alone is not a reliable indicator of the real surgical risk, but it must encourage the adoption of multidisciplinary collaborative models of care for this group of patients.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00268-023-06975-w.