As a Prince George's County man and his wife prepared for work each morning, they locked their doors and then locked his elderly mother in her bedroom so that the mentally impaired woman would not wander off or hurt herself.

It did not occur to the woman's son and his wife what would have happened if there had been a fire or other emergency, county health officials recounted recently after finding help for the family.

The elderly woman, Prince George's County health department officials said, was being abused, though not in the physical sense most people associate with the term abuse. She was a victim of neglect, a more insidious and difficult-to-prove form of abuse that researchers said affects elderly people nearly as often as the more widely publicized physical and sexual assaults affect children.

As the nation's elderly have become its fastest growing and most entrenched population group, the problems and concerns of the aged have edged their way onto national and local agendas. In the Washington area, the Council of Governments reports that the number of people aged 65 and over increased by nearly 32 percent between 1970 and 1980, to 227,000 people. By 1985, it was estimated, that number had grown to 263,000, or 8.4 percent of the region's population.

A U.S. House subcommittee estimated in a report last spring that abuse of the elderly affects one out of 25 Americans, or 4 million people over the age of 65 each year.

But despite the increasing interest in the elderly population, many of the activists and researchers working with their problems do not agree on how abuse can best be addressed. At the same time, programs to help the elderly are suffering from the same funding constraints affecting other social service programs.

In Prince George's County, for example, Betterment for United Seniors, which has proposed that the county build a shelter to house the abused elderly, is sponsoring sit-ins in the county executive's office and teach-ins at senior service centers.

BUS member Barbara Taylor said, "Once the abused elderly know there's a safe haven where they can come and compare experiences . . . they'll start to come out of the woodwork."

But no other local jurisdiction offers such a shelter, and Prince George's officials are reluctant to endorse it, fearing it will be expensive and unworkable for senior citizens who need a permanent place to settle and who have complicating illnesses.

The University of Massachusetts Center on Aging found in a report released in April that the typical victim of elderly abuse is likely to be female, 75 years old and dependent on family members for housing and support. The abuser, the study found, is often a younger male, and the most common form of maltreatment is psychological abuse.

The statistics available on the number of abused elderly are not conclusive. "We haven't been able to come up with good figures because so many go unreported," said Michael Shuster of the Legal Counsel for the Elderly in Washington. Other researchers report similar problems. "It's like looking at the tip of an iceberg," said BUS president Ralph Pryor. "You know very well there's a lot more under it."

In Maryland, a governor's task force on elderly abuse and neglect has found that abuse reports made to the state office on aging rose from 404 in 1983 to 594 for the fiscal year that ended in June. During that same period, complaints filed against nursing homes and other institutions in Maryland increased from 42 to 135.

If reports such as this are accurate, "elder abuse is only slightly less common than child abuse," Drs. George Taler and Edward Ansello of the University of Maryland wrote in American Family Physician magazine in August.

Sue Ward, director of Prince George's County's office on aging, is one of several local officials who suggests that instead of establishing a shelter, social service agencies should concentrate on developing programs such as day care for the elderly, which provides activities for older people during the day, providing relief for both the dependent senior citizens and family members responsible for taking care of them.

"A lot of the support service that could be provided for families who want to have elderly relatives living with them are nonexistent," she said. "All sources of domestic violence tend to hit on the frailest member. Often that person is the older adult."

Respite care, a program in which outside help is brought into the home to give the caretaker time off, is being implemented in some jurisdictions as an alternative to removing people from their homes. In Maryland, Gateway II operates in nine jurisdictions, including Prince George's and Montgomery counties, providing in-home care for frail elderly persons who otherwise would have to be placed in nursing homes. In Virginia, similar services are provided through six programs, ranging from adult day care to home-delivered meals.

Until recently, very little attention was paid to the three types of elderly abuse identified now by researchers -- physical abuse, neglect and financial exploitation -- because, social workers said, they were difficult to define and harder to keep track of.

"We're still at a point when the public is not very well educated," said Michael Shuster.

Last year, Maryland and the District of Columbia enacted laws requiring doctors, social workers and others who interact with elderly clients to report suspected incidents of abuse to government authorities. Virginia enacted a mandatory reporting law in 1977, and a bill that recently passed both houses of its General Assembly would for the first time establish penalties of up to $1,000 for repeat offenses of failing to report suspected incidents.

Maryland's law does not establish penalties for failing to make reports, but a bill pending in the General Assembly would establish a $5,000 fine if certain nursing home personnel fail to report.

A 24-hour phone line (727-2345) has been established in the District to accept abuse reports.

Unlike child abuse, the neglect, exploitation and physical abuse of old people cannot be detected in classrooms or on playgrounds. For years, much of the evidence was anecdotal rather than statistical, and experts struggled for solutions to a problem they could not prove existed.

"Mistreatment of these people can really be under the table for a long period of time before anything is found out," Ansello said.

Barbara Weideman, a member of Prince George's BUS who worked as an adult crisis counselor for the county's Department of Social Services until her retirement in 1983, said that awareness of the issue of elderly abuse is just the first step toward solution.

"You have an elderly person anywhere from 75 to 95 years old," she said. "Some are mentally incapable of saying what's going on. Some exaggerate. Some don't say anything."

Those who do not speak up are often "too sick or ashamed or dependent," said Pat Lusk of the Prince George's health department geriatric division. And sometimes, she said, the abusers do not realize that they are hurting the elderly person -- such as the Prince George's couple who locked up their mother.

The son said that they had confined the elderly woman to her room for her safety while he and his wife were earning the two paychecks needed to keep the family at subsistence level. They had no extra money to pay for day care or other services for the woman, he told county workers. With the help of county social workers, the family found a program it could afford for the elderly woman during the day.

"Yes, that's abuse," said Lusk. "But, no, that's not malicious abuse." Such neglect, Lusk and other experts said, is difficult to detect and nearly impossible to treat.

"It's not so much malicious abuse but the strain that occurs when you have people who might not even like each other living together," said Lusk. "It's always been there, but I'm seeing more of it with younger families and older folks being forced to live together for economic reasons."