President Jimmy Carter

Jimmy Carter is reportedly doing well, recovering from surgery for a broken hip sustained after the 94-year-old former president fell at his home. With falls among the top five leading causes of preventable death, context is paramount.
With President Trump's annual physical looming, what matters most? And specifically, what would it take to make a president unfit to serve? 
A statement released by Habitat for Humanity stated that the former U.S. president "was dehydrated working in the hot sun and ha[d] been taken offsite for observation." With that we ask, "Who is at risk of heat-related illnesses?"