Bone Loss a Serious Threat to Older Americans By LAURAN NEERGAARD    WASHINGTON (AP) -- Half of Americans older than 50 will be at risk of fractures from too-thin bones by 2020, the surgeon general warned Thursday, urging people to get more calcium, vitamin D and exercise to avoid crippling osteoporosis.    The bone-thinning disease is on the rise as the population grays - but weak bones aren't a natural consequence of aging, Surgeon General Richard Carmona stressed...