NEWARK, N.J. (AP) _ Ruth Guth came out of retirement at age 74 to fry hamburgers, joining a wave of older workers helping the fast-food industry through a nationwide shortage of teen-agers.

''We are between the baby boom and the baby boomers' kids,'' said Ben Morse, spokesman at Miami-based Burger King.

The fast-food restaurants are also recruiting the handicapped and housewives looking for part-time work.

Mrs. Guth went to work 32 hours a week at the Roy Rogers restaurant in Parsippany, N.J., five years ago after her doctors told her she needed to keep busy.

''How much can I do in my apartment?'' she said. ''It's something different. It keeps me occupied.''

Senior citizens make up between 5 percent and 10 percent of the 20,000 hourly workers at the 530 Roy Rogers restaurants nationwide, said Mary Maguire, a spokesman at the chain's Bethesda, Md., headquarters.

''Some of our business managers say they really enjoy having senior citizens working because they tend to mother the kids,'' said Ms. Maguire.

In past years, the expanding fast-food chains have looked to teen-agers to fill minimum-wage jobs such as cooking and cleaning up.

But a drop in the birth rate during the later 1960s means there are fewer teen-agers. Census figures show there were 27.3 million youths ages 15 to 21 in 1984, down from 30.5 million in 1980.

To close the gap with other workers, analysts say, the restaurants may have to serve up recruitment bonuses, higher wages and possibly more expensive hamburgers.

Burger King has been training its managers to tap alternative worker markets. ''In some cases we may turn to senior citizens,'' Morse said. ''Or we can turn to the homemaker who wants to earn a little extra income.''

At Roy Rogers, said Ms. Maguire, ''We are looking at young mothers, older students, people with flexible schedules.''

Dorothy Dee, a spokeswoman for the National Restaurant Association in Washington, D.C, said older workers are often more responsible than their younger counterparts. ''They are people who have come out of harder times and tend to be more reliable,'' she said.

Restaurants are also drawing increasingly on handicapped workers. Ms. Dee said a recent study by her agency showed the handicapped workers ''to be very reliable, as far as diligence and punctuality and other things that are very important to the employer.''

Women in their late 20s to 50s make up another source of workers, said Bryna Fraser, a senior programming officer for the National Institute for Work and Learning in Washington, D.C.

''There are women who have been out of the labor market, usually to have children, who are interested in re-entering the labor pool,'' she said.

At Roy Rogers, these women make up 10 percent to 20 percent of the workers, said Ms. Maguire.

Ms. Fraser said a recent study by the Bureau of National Affairs showed that 46 percent of fast-food managers report having turned to new recruitment methods to attract workers. One restaurant hired a full-time recruiter to hire hourly workers, she said.

Other methods include leaving job applications on the restaurant counter, posting openings on signs and marquees and going to churches, workshops for the handicapped and schools to find potential workers.

Ms. Fraser said one fast-food store in Massachusetts increased its hourly rate to $5.50, $2 above minimum wage. Others give cash bonuses to employees who bring in recruits and offer transportation to their workers.