Background: Research has shown that body MASS index (BMI) does not take into consideRATIOn the gender and ethnicity. The primary purpose of this study was to examine the interaction effect of the BMI and age on fat‑free MASS (FFM), waist‑to‑hip RATIO (WHR), and soft lean MASS (SLM). The secondary purpose was to evaluate the practical significance of the findings by examining effect sizes.Materials and Methods: The study was comparative in nature and employed a factorial design. Due to nonexperimental nature of the investigation, no causal inferences were drawn. The nonprobability sample consisted of 19, 356 adults. Analysis of the data included factorial analysis of variance, analysis of simple effects, calculation of mean difference effect sizes, and data transformation. The Statistical Package for the Social Sciences version 22 was employed for the purpose of data manipulation and analysis.Results: The BMI by age interaction effects on FFM, F (10, 19, 338) =28.26, P<0.01, on WHR, F (10, 19, 338) =18.46, P<0.01, and on SLM, F (10, 19, 338) =14.65, P<0.01, was statistically significant and ordinal in nature. Analysis of the effect sizes, ranging from 0.30 to 1.20, showed that the BMI and age influenced the WHR but their interaction effects on FFM and SLM, ranging from 0.04 to 0.36 and 0.03 to 0.33, respectively, were mainly negligible.Conclusion: Based on the examination of the statistical and practical significance of the results, it is concluded that the BMI and age together can influence the WHR but their interaction effect on the FFM and SLM is questionable.