Evaluation of body composition is important in the study of human energy and protein metabolism as methods are available for quantifying energy stores and protein content at a single point in time; energy-protein balance can be monitored over time; and dynamic measures of energy and protein metabolism can be referenced to body mass and related measurable components for between-individual comparisons. This review emphasizes the need for considering subject age when developing body composition component prediction models that are applied in elderly populations. An overview of body composition research is provided that emphasizes compartment and level definitions and interrelations. Two broad method categories, mechanistic and descriptive, are then critically examined in relation to their role in energy-protein metabolism and aging research. Our collective review indicates that all major body composition components are now measurable using one or more methods that are based on non age-dependent assumptions. We also found that some methods, particularly descriptive field methods (eg anthropometry), may be based on age-sensitive assumptions and measurements and suggestions for future development of these methods are provided. Lastly, as body composition differences between races, cultures, and countries are now recognized, it would be useful to create international cooperative groups with the aim of developing widely applicable descriptive field methods based on simple available techniques such as anthropometry and bioimpedance analysis.