TODAY, WHEN YOU ORDER A large Pepsi at your local Burger King, the cashier may hand you an empty cup to fill at the self-service drink machine. In 10 years, you will work harder for your meal. You may punch your burger order into a computer, pop the food into a microwave, and pay by credit card -- all with little human contact.

The dwindling number of young people is causing a shortage of low-wage labor, and the problem will get worse. There are 36 million Americans between the ages of 16 and 24. By 1995, the pool will shrink to 30.5 million, a 15% contraction. The shortage will affect such labor-intensive businesses as hotels and retailers, but the consequences will fall most heavily on restaurants, which draw over half their employees from the key age group. While its labor pool shrinks, the fast-food industry is expected to expand 2% to 5% annually.

Restaurants are attacking the problem with short-term solutions, such as flexible work hours and job applications on fast-food tray liners. But these approaches have limited effectiveness. "It's not something where you can have an image campaign and tell people how wonderful it is to work in a kitchen, because after they've been there two hours, they're going to know better," says Ron Paul, president of Technomic Consultants, in Chicago.

Modern fast-food chains were created by an earlier labor shortage. The current problem could change the industry just as dramatically. As wages rise, restaurants may splinter like gas stations into expensive, full-service operations and cheap, self-service "refueling" stops. Restaurants with few resources to invest in labor-saving equipment may be squeezed out of business.

Fast-food restaurants, whether franchised or independent, may rely on customers to put their own meals together. "If you want to see a sophisticated fast-food operation, go into a 7-Eleven," says Atid Kaplan, professor of hotel and restaurant management at the University of Denver. To buy lunch at a 7-Eleve, customers pick out their food, heat it in a microwave, and pay a cashier -- the only employee involved.