With a nudge from the new health care law and pressure from Medicare, hospitals, doctors and nurses are struggling to prepare for explosive growth in the numbers of high-risk elderly patients. More than 40% of adult patients in acute care hospital beds are 65 or older. Seventy million Americans will have turned 65 by 2030. They include the 85-and-older cohort, the nation’s fastest-growing age group. Elderly people often have multiple chronic illnesses, expensive to treat, and they are apt to require costly hospital readmissions, sometimes as often as 10 times in a single year.