What do you say to a teenager with a job? Answer: "A Big Mac, fries and a Coke, please."

At least, that was the joke in happier times. Today's question might be: "What do you say to a single parent with two jobs and no health insurance?"

The old stereotype that burger-flipping is how teenagers earn pocket money is no longer true, if it ever was. Half of fast-food workers during 2010-12 were aged 23 or over, according to John Schmitt and Janelle Jones of the Center for Economic Policy Research, a think tank. Only 30 percent were teenagers; 1 percent of them were 65 or more. Of the non-teenagers, around 85 percent had graduated from high school, and more than a third had some higher education. More than one in three is bringing up at least one child.

On Aug. 29, shortly before Labor Day, protests were staged outside more than 1,000 fast-food restaurants in 60 U.S. cities. Protesters called for the minimum wage to be raised to $15 an hour, up from $7.25 (the federal rate; some states are higher.) "Fast Food Forward" has grown rapidly since last November, when 200 New York workers went on strike for higher pay and unionization.

About 13 percent of fast-food workers made roughly the minimum wage in 2010-12; another 70 percent earned between that and $10.10 an hour. The minimum wage has not changed since 2009. After adjusting for inflation, it is worth less than three-quarters of its peak value in 1968, says the Congressional Research Service. The decline relative to average wages is even more sobering. In 1968 the minimum wage was 54 percent of average hourly wages in the nonfarm private sector; this year it is 36 percent.

The protests mark the start of a nationwide movement that "has touched a nerve in America about growing inequality," says Mary Kay Henry, president of the Service Employees International Union (SEIU), which has helped coordinate them. She adds that protesters are winning support from politicians, religious leaders and other poorly paid workers.

The proportion of private-sector workers who are unionized has tumbled from 25 percent in 1973 to 7 percent. The SEIU points to successes in organizing low-paid service workers such as caretakers and security guards. Union leaders urge companies to look at the benefits of higher wages, like reducing turnover.

Until recently, hardly anyone tried to organize fast-food workers, because none of them intended to stay in a McJob for long. That this is changing is a "sign of how desperate things have become for low-wage workers," said Schmitt.

Yet waving placards is unlikely to raise wages much. Although unemployment has fallen from a peak of 10 percent in 2009 to 7.3 percent today, there are plenty of jobless Americans available to serve the burgers the strikers won't. And if the cost of labor rises, firms might use less of it. "The burgers of tomorrow could be made by robots," warns the Employment Policies Institute, which is funded by business.

"It is going to be hard for the fast-food workers to succeed," said union organizer Jonathan Lange, "But if they could get one of the fast-food companies to break ranks, that would help."