WebMD says that a six-pack cannot be achieved by doing just abdominal exercises such as sit-ups. The key to getting six-pack abs is to remove the fat layer over the abdominal muscles, which requires proper nutrition and cardiovascular exercise to burn calories.
For the abdominal muscles to be visible, the body fat level needs to be reduced to levels that are unrealistic for women of any age and men older than 30. Body fat levels low enough to show a six-pack may be so low that women stop menstruating.
In addition to dieting and performing cardiovascular exercise, WebMD recommends performing abdominal exercises two to three times a week.