BALLACHULISH, Scotland—At the windswept summit of Buachaille Etive Mòr, a Scottish peak more than 3,350 ft)above sea level, 81-year-old Nick Gardner stops to gaze down at the rugged peaks, lochs, and valleys visible through the drifting clouds. “It’s so exciting every time I still feel like a little boy,” he said. “I need my hat and gloves on, and my windproof jacket, but it’s absolutely magnificent. I’d much sooner be here than in a centrally heated house.” For Gardner, who moved with his wife Janet from suburban England to a remote corner of the Scottish highlands at the age of 50, climbing mountains has become more than just a boyhood dream. Two years after being diagnosed with Alzheimer’s in 2018, Janet, a former teacher who also suffers from osteoporosis, became so ill she needed 24-hour care. Gardner was devastated and decided he needed a challenge to help him cope. “When …