For a woman, the bodyfat ranges provided when doing diagnostics are 18%-26%. Lady C is in exceptionally good shape and exceptionally above-average with a beautifully
lean body. Her bodyfat is comparable to that of a teenage cheerleader.
Bodyfat above 26%-28% is considered over-fat, or obese, and unhealthy, and (statistically) puts the woman at greater risk for health problems.
26% is statistically considered "average."
18% is statistically considered "acceptable" for very young women, teenagers, and athletic, highly active women. It's usually extremely difficult for an adult woman to attain this low level of bodyfat, or as Lady C pointed out, to maintain that degree of leanness over time.
It can be done, but it mostly depends on your genetics, lifestyle, activity, diet, etc.
Remember, too, that when getting BF%, it is not the same thing as BMI.