As a teenager in South Korea , Ms. Chung trained in ballet, even dancing on point but never professionally. Now, she scours YouTube for choreography ideas. She might “see something that our knees can handle,” Ms. Chung said. “I’ll think, that looks good. I listen to the music and I practice on my own. I study gestures and movements a lot. I also have to keep in mind the condition of the dancers, because they are a bit older so they can’t do anything too crazy like spinning around a bunch. Otherwise they’ll get dizzy.”

Of course, age creates physical limitations. But there is artistry in their dancing and musicality, in the way they hang a fraction behind the beat to create the lilting sensation of floating. It’s soulful. By the end of their sessions, which do involve breaks — cookies and coffee are essential for recharging the body — they seem to transform into lighter, younger versions of themselves.

You wouldn’t know that Susan Lee , a graceful wisp of an 84-year-old, has had two knee replacements, wears a pacemaker and is diabetic, which affects the vision in her left eye. Even when walking hurts, she said, “Dancing helps me feel better.”