To have a toned mid section you often need lower body fat, and higher muscle mass/ definition.
Cut out most of the high sugar, highly processed foods in your diet. The average american consumes 130 - 150 lbs of sugar a year. Make your diet 70% fruits, vegis, and 100% whole grains . The other 30% can be lean meats, fish, un- salted nuts, beans, and even dairy.
The next part is exercise. Lift weights 3 times a week, and stick to mainly Deadlifts, Sqauts, Over Head Press, Bench Press, Leg Press, Dips, and Pull Ups. Aim to get stronger in these workouts, and you will increase your lean tissue. Increasing your lean tissue will increase your mettabolism, and encourage your body to produce more hormones that are pro fat burning.
High intensity cardio is also important. Instead of the long hour jogs on the treadmill. Think of doing Hill Sprints, or peddaling on the stationary bike on high restistance, as fast as possible for 12 minutes every day. Higher intensity cardio saves time, can promote fat loss, and promote lean tissue gain. 10 hill sprints on a 30 yard hill burns as many calories as 45 minutes jogging on a treadmill with no incline.
Crunches may increase the muscle of your abdominal/ mid section. However most people do not know that to see your abs, you need lower body fat. IF your body fat is low enough your mid section may look toned even without doing crunches. However as a female you need to stay over 15% body fat for survival and reproduction purposes. Ask your doctor as well.
Many people gain a (beer belly) or more body fat later in life as they become less active, more sedantary, and certain hormones that keep you lean , and dictate growth start to decline in both the late 20's, and early 30's. You can stay more toned looking with lifestyle changes good luck.