Seriously, why? Bone is a composite, of apatite that provides rigidity and collagen which provides toughness. Bones of children are soft. They do fracture under excessive forces, but are liable to greenstick fractures. So soft bones of children may deform plastically, as in rickets. Both rickets and osteoporosis are caused by not enough apatite in bones. Why do adult osteoporotic bones fracture, rather than deform plastically?