A person's basal metabolic rate (BMR) is determined by several factors including age, sex, weight, height, and body composition. As people age, BMR generally declines due to a decrease in lean muscle mass, which is metabolically active. Males typically have a higher BMR than females due to greater muscle mass. Additionally, taller individuals or those with a higher weight tend to have an increased BMR, as they require more energy to maintain bodily functions. Understanding BMR helps tailor nutrition and exercise plans for effective weight management and fitness goals.