No refund necessary--so far. By the time you're 18, you should have about 92 percent of the bone mineral content for all the bones in your body. And though your bones are constantly remodeling themselves, they can't add much to what has been added in these crucial teen years. After about 30 or so, the amount of bone you've stored will begin to steadily drop. So be sure you're getting the right amount of calcium now. Your bones will thank you later in life.