Purpose: This study aimed to identify risk factors for complications and failure following plate osteosynthesis for PHF and discuss a cut-off age that surgeons should consider the high rate of complications. Methodology: Adults with PHF treated with proximal humerus internal locking system (PHILOS) at our tertiary center were identified. Univariate analysis was used to compare complicated and non-complicated patients. Then, a multivariate regression analysis was conducted to assess the risk factors of complications. Results: Complications were observed in 19 cases (14.1%) among 135 patients, including nonunion (n=5), avascular necrosis (n=1), infection (n=1), intra-articular screw perforation (n=1), arthritic changes (n=5), and superior humeral head migration (n=5). In logistic multinominal regression analysis, only older age (OR=1.09) and Neer IV class (OR= 8.3) were associated with an increased likelihood of complication (P<0.05). The age ROC curve for predicting complication-free cases had an area under the curve (AUC) of 0.829, indicating that age could predict the outcomes of ORIF in PHF (P<0.05). The optimal age cut-off was 44 years, which yielded a sensitivity of 100% and a specificity of 65.7. The age ROC curve for predicting favourable Oxford shoulder score (OSS<10) had an of 0.829, indicating that age could predict the favourable OSS outcomes of ORIF in PHF. The optimal age cut-off was 46 years, yielding a sensitivity of 70.0%, and a specificity of 78.6%. Conclusion: The outcomes of ORIF for PHF in patients older than 45 years are associated with significantly higher complication rates and inferior functional outcomes.