WASHINGTON – Too little milk, sunshine and exercise: It's an anti-bone trifecta. And for some kids, shockingly, it's leading to rickets, the soft-bone scourge of the 19th century.

But cases of full-blown rickets are just the red flag: Bone specialists say possibly millions of seemingly healthy children aren't building as much strong bone as they should – a gap that may leave them more vulnerable to bone-cracking osteoporosis later in life than their grandparents are.

"This potentially is a time-bomb," says Dr. Laura Tosi, bone health chief at Children's National Medical Center in Washington.

Now scientists are taking the first steps to track kids' bone quality and learn just how big a problem the anti-bone trio is causing, thanks to new research that finally shows just what "normal" bone density is for children of different ages.