During childhood and adolescence, the body uses calcium to build strong bones—a process that's all but complete by the end of the teen years. Getting enough calcium at a young age is important to prevent osteoporosis later on. Yet more than 85 percent of girls and 60 percent of boys aged 9 to 18 fail to get the recommended 1,300 milligrams of calcium per day (kids aged 4 to 8 years old need 800 mg; toddlers aged 1 to 2 years need 500 mg). The 2005 Dietary Guidelines recommend 2 cups of low-fat or nonfat milk or dairy products per day for children 2 to 8 years old and 3 cups for children 9 years and older.