92 to 98 percent of peak bone mass (the period by which all bone formation occurs) is achieved by age 20.
High peak bone density is one factor that helps decrease the risk of osteoporosis and bone fractures later in life.
This is precisely why having adequate intakes of calcium -- as well as being physically active -- during childhood and adolescence is crucial.
The more "bone healthy" the diet is during childhood and adolescence (particularly by consuming sufficient amounts of calcium, vitamin K, vitamin D, phosphorus, and magnesium), the higher peak bone mass levels are.
This is not simply a matter of having your child pop a Flintstones chewable vitamin, though. Whole foods containing these nutrients are far superior sources.