Procter & Gamble Co. is getting back into a business it exited more than a decade ago—making products for adults suffering from incontinence—as it takes aim at the growing ranks of aging Americans.

Births peaked in the U.S. at 4.32 million in 2007 and declined for five years before leveling off recently. Some 3.96 million babies were born in the U.S. last year, according to preliminary data from the Centers for Diseases Control and Prevention.

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