Yul Kwon rises at 6am and starts his day with stretches and a fitness regimen that includes planking for 60 to 90 seconds, 50 sit-ups, 50 squats, and standing on one leg for a minute and then the other – twice. He then eats a banana and, every other day, he heads out for a 10km (6.2-mile) run. That is an impressive regular warm-up for a 90-year-old. “Health is most important, so I put priority on my daily exercise,” says the sprightly Korean-Canadian, a retired economics professor living in...