At what age does a woman's metabolism start slowing down (aside from factors like physical activity, diet, etc.)?

Let try to explain: Metabolism is the name for the process your body uses to convert the food you eat into energy. The energy your body does not use is stored as fat. Many people believe that this process slows as you age, causing weight gain. However, this is a misconception, since your metabolism is more directly related to your level of activity throughout your life.