Objectives: Our study aimed to investigate the association between body mass index and clinical outcomes by age group in a population-based cohort of patients having HD. Methods: The data collected by the Health Insurance Review and Assessment Service during HD quality assessment programs and the claims and death data were retrospectively analyzed (n = 71,382). The patients were divided into four groups according to their age. Each group was further divided into four groups in accordance with previous guidelines: underweight group; normal group; overweight group; and obese group. Results: The age groups were composed of the following: 41,395 patients aged <65 years, 18,329 patients aged 65–74 years, 10,354 patients aged 75–84 years, and 1,304 patients aged ≥ 85 years. In all age groups, the all-cause mortality of the underweight group was higher than that of the normal group. Multivariable Cox regression analyses revealed that the cardiovascular event outcomes of the patients aged < 65 years and those aged 65–74 years in the overweight and obese groups were better than those in the normal group. However, in patients aged ≥ 75 years, the association between body mass index and cardiovascular event outcomes was attenuated. Conclusion: Low body mass index, was associated with poor patient survival irrespective of age. Conversely, patients in overweight or obese groups had modestly lower or comparable all-cause mortality relative to those with normal body mass index. In patients aged < 65 years, patients with low body mass index showed a higher risk of cardiovascular event compared with those with normal body mass index.