Background: Elderly surgical patients have a high risk of postoperative complications. However, patients exhibit considerable diversity in health and functional status; thus, identifying the fragile may be necessary when selecting surgical candidates. We aimed to compare the prevalence of frailty in patients ≥90 years with patients aged 80–89. Second, we assessed the association between frailty and all-cause 30-day mortality. Methods: We performed a planned secondary analysis of the peri-interventional outcome study in the elderly (POSE), including 9497 patients (≥80 years) undergoing any surgical and nonsurgical procedures in 177 European centres from October 2017 to December 2018. The primary outcome assessment included frailty as a binary variable, and data were analysed using Fisher's exact test/Chi-squared test. The association between frailty and all-cause 30-day mortality was analysed using a multivariate logistic regression model adjusted for age, sex, surgical urgency, orthopaedic urgency, and surgical severity. Results: In total, 999 of 9497 (10.5%) patients were 90 years or above. Among patients ≥90 years, 274 (27.4%) were frail compared to 1062 (12.5%) of patients aged 80–89 (odds ratio (OR): 2.6; 95% CI 2.3–3.1). Frailty was associated with increased 30-day mortality in both the unadjusted (crude OR 6.3; 5.1–7.7) and adjusted analysis (OR 4.5; 3.6–5.7). In the adjusted analysis, age ≥90 was not associated with 30-day mortality. Conclusion: We found a high frequency of frailty in patients aged 90 years or above compared with patients aged 80–89. In addition, frailty was associated with an increased risk of 30-day mortality. Surprisingly, age was not a significant risk factor in the adjusted mortality analysis.