Call it a sign of the times: The number of knee replacements in older adults shot up by a dramatic 46 percent from 2009 to 2013. Chalk it up to an aging population. Obesity. Sports injuries. All contribute to debilitating knee pain — and the growing demand to do something about it. “Getting old, age-wise, is not a sentence to not be active,” said Dr. Ravi Bashyal, an orthopedic surgeon at NorthShore Orthopaedic Institute in Evanston. One of his patients, Andrea Siegel, had both knees replaced last year.