Objective To explore the effects of low body weight and different ages on the 28-day death and other clinical outcomes of intensive care patients. Methods Data were extracted from Medical Information Mart for Intensive Care Ⅲ(MIMIC-Ⅲ) intensive care database, and adult patients admitted to the intensive care unit (ICU) were screened. The body mass index (BMI) is calculated according to the initial weight and height when entering the ICU. According to their age, they were divided into two groups when they entered the ICU: the younger age group <65 years old, and the older age group ≥65 years old. BMI was divided into two groups: low BMI group <18.5 kg/m² and non-low BMI group ≥18.5 kg/m². Univariate analysis, multivariate logistic regression, and interaction analysis analyzed the impact of low body weight and different ages on the 28-day death and other clinical outcomes of intensive care patients. Results A total of 17 134 patients met the inclusion criteria. According to their age, they were divided into 7986 patients in the low-age group and 9148 patients in the high-age group. According to BMI, they were divided into 458 patients with low BMI and 16 676 patients with non-low BMI. Compared with non-low BMI, patients with low BMI were older(66.65±18.02 vs. 64.73±15.98, P=0.012), had a higher proportion of recent weight loss (19.7% vs. 3.7%, P<0.001) and emergency admissions (83.0% vs. 73.0%, P<0.001), had a higher prevalence of chronic obstructive pulmonary disease (COPD), rheumatoid arthritis, cancer, sepsis, and acute renal insufficiency. However, patients with low BMI had a lower prevalence of hypertension and diabetes. On the final clinical outcome, patients with low BMI had a higher prevalence of sepsis and acute renal insufficiency and longer mechanical ventilation time [29.2(12.0, 106.0) h vs. 18.7(5.7, 93.8) h, P<0.001]. In the whole cohort and different age groups,it could be seen that the 28-day mortality rate of patients with low BMI was higher than that of non-low BMI. Univariate analysis showed all the characteristics, including high age, low BIM, weight loss, emergency hospital admission, cardiac insufficiency, COPD,cancer, sequential organ failure assessment (SOFA) score, acute renal insufficiency, sepsis, septic shock, and highest white blood cell,were risk factors for 28-day death. Multivariate analysis showed that cancer and SOFA score were independent risk factors for 28-day death. In the interaction analysis, compared with the low-age group with non-low BMI, after adjusting the variables of P<0.05 in the multivariate regression and clinically significant variables, the low-age group with low BMI (OR=1.77, 95%CI 1.24-3.14, P=0.004),high-age group with non-low BMI (OR=1.84, 95%CI 1.64-2.08, P<0.001) and high-age group with low BMI (OR=2.95, 95%CI 2.09-4.17, P<0.001) were risk factors for death at 28 days. Conclusions Low BMI is a risk factor for short-term death in intensive care patients of different ages. Its poor prognosis may be related to low BMI patients with high cancer prevalence, severe illness, and longer mechanical ventilation time.