Most research finds no correlation between caffeine consumption and bone growth in kids.

In adults, researchers have seen that increased caffeine consumption can very slightly limit calcium absorption, but the impact is so small that a tablespoon of milk will more than adequately offset the effects of a cup of coffee.

Advertising seems to be largely responsible for this myth: Cereal manufacturer named C.W. Post was trying to market a morning beverage called "Postum" as an alternative to coffee, so he ran ads on the "evils" of Americans' favorite hot beverage, calling it a "nerve poison" that should never be served to children.