CHAPEL HILL, NORTH CAROLINA — At the age of 17, Holden Thorp placed fifth in a nationally televised Rubik’s cube competition on the ABC show That’s Incredible! At 24, he received a doctorate in chemistry from the California Institute of Technology after studying for three years instead of five. And at 43, he was named chancellor of the University of North Carolina at Chapel Hill, becoming one of the country’s youngest university presidents.