Great article in Women's Health section of Web MD breaks down the type of workout you need for each decade in your life.
Your 20's are when you are laying the foundation for your future in terms of your bone density, muscle strength, and cardiovascular health. "So the more exercise, the better." During your 30's is when many women find it hard to find the time to exercises because of the increase demands of work, and family, and it is usually when they are faced with "baby weight". Try to exercise first thing in the morning to ensure you get the time to work out and do a lot of core exercises to bring back those abdominals.
By the time you hit your 40's your metabolism starts to slows down and muscle mass starts to decrease; Progressive weight training and weight bearing exercises, such as walking, will help maintain muscle mass and prevent osteoporosis. Fitness training for women in their 50's includes increased light cardiovascular training along with weight training. Continue all of the above for the rest of your life and studies have show that you will live longer.