The U.S. minimum wage, $7.25 an hour, hasn’t been raised since 2009. But 29 states and the District of Columbia and at least 53 cities and counties have lifted their pay floors above the national mandate. So have a number of corporations including Walmart Inc., which now offers minimum starting pay of $11 an hour, and Amazon.com Inc., which raised its minimum wage in the U.S. and the U.K. to $15 an hour.