<qa>  What is the minimum age of subjects to be between the ages of 8 and 21 years?- Subjects must be between the ages of 8 and 21 years, 11 months, at randomization. Although subjects younger than 8 years of age may be affected by osteoporosis, limited normative data prevents assignment of a BMD Z score for this group. Subjects through 21 years, 11 months will be included because many individuals with chronic disorders will have an immature skeleton, and even in healthy individuals there is significant accrual of bone mass into the 20s, making this a dynamic and critical time period for analysis. 