Over the years, body fat deposits tend to get redistributed. At the same time, the proportion of fat around the mid-section, hips and thighs increases. Even aging marathon runners who may not have gained an ounce since their 20's, tend to develop at least a bit of a belly with age. Tummy fat, like other subcutaneous adipose tissue, rides on top of muscle. Fat provides energy for fuel, but fat does not play an active part in performing exercise.
Muscles do the work that causes the movement.
Also, for any specific exercise, fuel is drawn from deposits throughout the body, not just from the fat adjacent to the active muscles. Still, the following exercises will tighten-up your stomach muscles to help give you a trimmer look. And, of course, you will also burn some calories in the process. Start with your back really flat on the floor. You shouldn't be able to slide your hand between your lower back and the floor.