Harley-Davidson Inc. said on Tuesday it will close a plant in Kansas City, Missouri, as it consolidates manufacturing operations after its motorcycle shipments fell to their lowest level in six years, sending its shares down more than 8 percent.

The Milwaukee, Wisconsin-based company forecast a drop in shipments to dealers this year as it expects retail sales in the United States — the company's biggest market — to dip.

Despite generally higher U.S. consumer spending, Harley is grappling with an aging customer base and younger, more price-sensitive buyers hesitant to embrace the iconic brand as previous generations have done.