A new McDonald's Corp. commercial tells the story of Karen King, who began her career as a teenage crew member in the 1970s and rose to head the company's $10-billion Eastern-U.S. division. The spots are meant to resonate with American teenagers, who are leaving the workforce in droves — and leaving McDonald's with a labor crunch that threatens to take a bite out of its surging sales. "It's a shrinking labor market, and we recognize less people will be available to hire," Ms. King says. The declining number of teenage job-seekers presents a super-size challenge for McDonald's, where 40% of...