Patients with oesophageal squamous cell carcinoma (ESCC) generally have distinctive body compositions; being underweight is highly prevalent and sarcopenic obesity is rare. We investigated the survival impacts of body mass index (BMI) in elderly (≥65 years) and non-elderly patients undergoing surgery for ESCC. In total, 379 ESCC patients were retrospectively reviewed. Patients were divided into 3 groups according to BMI; low (<20), medium (20-25) and high (≥25). The skeletal muscle index (SMI) was calculated and its relationship with BMI was analysed. Univariate and multivariate Cox hazards models were applied to determine independent predictors of poor overall survival (OS) and cancer-specific survival (CSS). The low-, medium- and high-BMI groups included 102 (26.9%), 231 (60.9%) and 46 (12.1%) patients, respectively. High BMI with low SMI was rare (n = 6, 1.6%). Patients with low BMI had significantly poorer OS and CSS than those with high and medium BMI (OS; P < 0.001, CSS; P = 0.003). Notably, OS and CSS curves were well-demarcated by BMI (both P < 0.001) in elderly patients, while not being stratified according to BMI in non-elderly patients (OS; P = 0.08, CSS; P = 0.54). Multivariable analysis revealed low BMI, as well as pStage III disease and non-curative resection, to be independent predictors of poor OS (HR 2.73, P < 0.001) and poor CSS (HR 2.88, P < 0.001) in the elderly group. The survival and oncological impacts of low BMI were evident only in elderly patients with ESCC. Our findings highlight the age-dependent significance of BMI in patients with this tumour entity.