Pediatric bone specialists know that the skeletal characteristics acquired in the adolescent and teenage phases of development are carried forward into adulthood. Stronger and bigger teenage bones beget stronger and bigger adult bones. A thicker teenage sub-cartilaginous bone layer — under the hip joint cartilage, for example — acquired through the stress of loaded work, play, and exercise is a thicker adult sub-cartilaginous bone layer, and a hip that is more resistant to osteoarthritis than that of a lazy kid/sedentary adult.
It has been my experience that most practicing pediatricians don’t know this. Most pediatricians advise children and their parents that kids should avoid lifting weights, under the pretense that it damages young joints or, for God’s sake, stunts the growth.
I once had a kid — a large, not-very-explosive kid — who was told by his pediatrician, “I’d hate to see you jeopardize your career in sports by lifting a bunch of heavy weights.” This is a comically tragic miscarriage of professional authority, and very bad advice.