Several factors influence resting metabolic rate (RMR), including age, gender, body composition, and genetics. As individuals age, muscle mass typically decreases, which can lower RMR since muscle tissue burns more calories than fat tissue. Males generally have a higher RMR due to larger body size and more muscle mass. Additionally, genetic factors can dictate how efficiently an individual's metabolism operates. Factors like hormonal levels, particularly thyroid hormones, can also significantly impact metabolism and energy expenditure.