Leaning on a cane or the barre in her Santiago dance studio, Evelyn Cordero lifts an arm and gently corrects the posture of her ballet students. "Even in my dreams, I dance," says the 100-year-old.

If you feel like you dance with two left feet, wait until you meet Robert "Bobby" Barnett, who turned 100 on May 6 and is still in his dancing shoes. Barnett, who grew up in Washington state and was ...