Backgrounds and aims To examine the contribution of patient body mass index to functional status, physical independence and emotional distress in various age groups (third and fourth age) of female hip-fracture patients. Methods A sample of 123 older females (>65 years) admitted in a major regional hospital with a diagnosis of hip fracture participated in this cross-sectional study. The outcome measures used in this study were body mass index (BMI), the Modified Barthel Index, the Goldberg General Health Questionnaire, the Tinetti Mobility Test and a survey collecting data from participants 24-72 h after admission. For our analysis, patients were divided into two groups according to their age: <80 years (third age) and >81 years (fourth age). In addition, three groups were made according to patients’ body mass index <24 h prior to surgery: a normal weight group, an overweight group and an obese group. An ANCOVA was performed with age group as a between-subjects variable (third age, fourth age) and gender, educational level, marital status, type of fracture, type of surgery, presence of other fractures and BMI as covariates. Results Patients in the third-age group obtained significantly higher values in the Barthel Index (P = 0.040) and the Tinetti Mobility Test (P = 0.001) and lower values in the Goldberg General Health Questionnaire (P = 0.035) compared to the fourth-age group. When BMI was considered, significance was maintained only in the Tinetti Mobility Test. Conclusions The BMI could be a relevant mediator of the relationship between functional decline and the aging process in the transition between third to fourth age in females.