This is a bad thing. Bone density is how thick your bones are. It is determined primarily between menarche and thirty years of age. Your bone density will partly determine whether or not you get osteoporosis, and how badly if you do. So between the ages of puberty and thirty is a critical age for women to get sufficient calcium, exercise, and to prevent low estrogen levels.
Proper nutrition is one path needed to arrive at healthy bones, and exercise is another.
Exercise, especially weight bearing exercise, can increase bone density in women at any age. Exercise done between puberty and thirtyish will help your body initially make thicker bones with which you can defend yourself against osteoporosis. Weight bearing exercises include activities like walking and running. Bicycling and swimming, while excellent aerobic activities, do not include the use of one s own body weight and do not force your body to increase the density of your bones as a training adaptation nearly as well. Weight lifting can work too, but not as well as walking.