With one of their arms raised and the other lowered, hundreds of people move every morning like birds spreading their wings at the heart of Beijing’s Temple of Heaven. They are performing a movement in tai chi, a physical and philosophical practice developed more than 300 years ago that continues to resonate in China today. “The environment is great and the air is good too,” Ye Guirong, 64, says. “You can see we’re surrounded by trees.” Most practitioners are retirees in their 60s and beyond....