The Apple Inc. designer who helped develop the iPhone Air — and even starred in its introduction video — has left, marking the latest setback for the company’s design group.

Cook's most likely successor is John Ternus, Apple's current senior vice president of hardware engineering. Ternus joined Apple in 2001, and at the age of 50, is one of the company's younger senior executives. Coincidentally, Cook was also 50 when he took over as the chief executive.