Methods: This study included 539 patients diagnosed with spinal metastasis from February 2009 to July 2018. The patients were categorized into four groups based on age: <59, 60–69, 70–79, and ≥80 years. Differences in trunk muscle composition among age groups and among groups were classified on the basis of survival (< or >3 months after spinal metastasis diagnosis) were evaluated. Results: In total, 515 patients (273 men, 242 women; mean age, 67.8 years) with complete medical records were included in the analysis. No significant differences were observed in the area of the psoas and paravertebral muscles among age groups in either sex. A significant trend toward a low muscle density with the increase in age was found for both sexes. Patients who survived less than 3 months had significantly smaller trunk muscle area than those who survived for more than 3 months in both sexes. Conclusions The results suggest that the reduction in muscle density is associated with advanced age, whereas a decreased muscle area is associated with pathology. Additionally, a small trunk muscle area was associated with a short overall survival. Further studies are needed to elucidate the underlying mechanisms of age- versus cancer-related changes in the muscle area and their influence on overall survival.