This is due to the fact that during infancy and childhood there are many growth spurts requiring high amounts of calories. After you reach 30, your BMR slows down by a rate of one to two percent a year. It becomes increasingly harder to lose and keep weight off as you grow older. Another significant factor is the influence of your body composition. Muscle tissue is highly active even at rest.
Men usually have a higher BMR because of their greater tendency toward lean muscle tissue.
Hormones also play an important part in basal metabolism. The BMR is likely to change during puberty and pregnancy due to hormonal changes. According to a recent USDA study, eating foods high in zinc may help prevent fatigue and increase endurance. To keep zinc up and fat down, eat lean cuts of red meat.