Behind McDonald’s Corp.’s worst slump in a decade is a trend that may auger even tougher times ahead: The Golden Arches is losing its luster with younger consumers. In the U.S., with more than 40% of McDonald’s 35,000-plus global locations, sales at restaurants open at least 13 months have been flat or falling for most of the past year. McDonald’s stock has traded in a relatively narrow range in the nearly 26 months since Chief Executive Don Thompson took the helm, while the share prices of many of its rivals have soared.