Factors contributing to an individual's basal metabolic rate (BMR) include age, gender, body composition, and overall body size. Younger individuals generally have a higher BMR due to increased muscle mass and growth demands. Men usually have higher BMRs than women, attributed to larger muscle mass and lower fat percentage. Additionally, BMR increases with more muscle mass since muscle burns more calories at rest than fat. Genetic factors also play a role, influencing metabolic efficiency and how the body processes energy.