From the Manufacturer:
Calcium is one of the most abundant minerals in the body. During childhood and adolescence calcium is used to build strong bones and teeth. Additionally calcium is an important factor in proper muscle contraction, the transmission of messages through nerves, and the release of hormones. Getting sufficient levels of calcium during these formative years is important for several reasons. First, getting sufficient calcium during the pre-adolescent years can reduce the risk of developing osteoporosis later in life. Secondly, if the amount of calcium taken in is too little the body will pull calcium from the bones to support proper cell function resulting in weaker bones.