They are the fastest growing poverty group in America. Among the hardest hit by inflation and the most discriminated against in employment, they have the best chance of dying alone, in nursing homes.

They are older women: our mothers, our sisters, ourselves.

"The problems of old age in America are largely the problems of women," says Dr. Robert Butler, director of the National Institute on Aging. "The world as a whole is graying, and women are living longer than men."

"Women in the 65-and-older age group are the fastest growing segment of our population," adds Nancy King, co-director of the National Action Forum for Older Women. "And they are the single poorest group of people in America.

"Today one woman in four outlives her eldest son," she says, noting that medical advances, particularly in reducing death during childbirth, have lengthened the average female life expectancy. A baby girl born today can expect to live to age 81.

There were 13.9 million older women and 9.5 million older men in 1977. Predicted for the year 2035 are 33.4 million older women, compared to 22.4 million older men.

King also cites these grim statistics:

About half of the 5 million older women who are living alone have yearly incomes of $3,000 a year or less. For black women the figure is $2,000. The poverty rate for older women is about 65 percent higher than it is for older men.

The median weekly salary for full-time working women aged 45 to 54 was $146 in 1975, and $118 for working women 65 and older. For older women not in the labor force (90 percent of women over 65), the median weekly income was $67.

Twenty-five percent of all women who are working now can expect to be poor in their old age. Middle-aged and older full-time homemakers can expect to live their later years alone and in poverty.

Three-fourths of all nursing-home residents are women. An estimated 50 percent of all nursing-home residents have no close family members alive.

The average age for widowhood in America is 56. Two-thirds of all widows live alone, and one-third live below the official poverty level.

The number of older women who are divorced is growing. One in two marriages ends in divorce and at least one-sixth of all divorces are now occuring in the over-45 age group.

"A major reason for poverty among older women is the built-in sex bias of our Social Security system and private penson programs," says King. Women receive less in Social Security benefits because they "disproportionately receive lower wages, often in part-time jobs, in careers which are discontinuous because of time taken for homemaking and childrearing.

"When a woman's contribution to society is calculated to determine her Social Security benefits, a zero is averaged in for the years she spent at home in a career which was socially designated and approved."

And benefits are calculated in a way that penalizes dual-income households, says the Congressional Budget Office's Nancy Gordon. If one spouse earned all the couple's income, and he or she dies, the surviving spouse, says Gordon, "can get up to 60 percent more in benefits than if that same income had been earned equally by two spouses paying the same amount of Social Security tax."

"The retirement systems in America punish women for combining work and family responsibilities," says Rep. Pat Schroeder (D-Colo.).

"Seventy percent of working women work between the ages of 18 and 25, yet pension eligibility begins at age 25. Most women leave the labor force at age 25 to raise children, and return between the ages of 35 and 44, yet the 10-year (pension eligibility) period must be with the same employer." (The result: Just 21 percent of women in the private labor force are covered by pensions.)

"Women who work in the home won't necessarily receive a share of their husband's pension if they divorce." And when a man dies, says Schroeder, his pension dies with him.

The women who can look forward to this dismal fate are those in mid-life now, notes King, who is writing a handbook for women over 40. The biggest problem, she says, is role loss.

"Most of today's mid-life women followed traditional roles: raising children and homemaking. By the time they reach age 40 this job is over -- yet they have two-thirds of their adult life left to live.

"They often have no job skills and lack education. And they live in a society where age and sex discrimination are major obstacles. At an age where a man is at the peak of his economic opportunity, a woman is seen by employers as a 'has been' who must compete with younger workers for low-paying, dead-end jobs."

Letters from hundreds of women over 40 express the same theme: "Nobody ever told me it would be like this,'" says King. "That's why I'm trying to get women in their middle years to make plans.

"It's never too early to think and plan for your older years. You've got to look at all your options: paid work, volunteer work, going back to school. And you've got to plan for other needs, too: good health, leisure, intimacy, affection."

Too many mid-life women have based their existences on a single relationship -- usually husband, parents or children -- notes social worker Marilyn Pearlman, who leads self-help and counseling groups for "women in the middle."

"But if they develop multiple anchors early enough -- work, friends, community -- they won't be devastated when one anchor breaks. They may find that mid-life is a time that can be very freeing and dynamic, instead of frightening."

The challenge for women over 40 is to "create new roles, new relationships, new life styles which enable them to meet all their needs throughout their lives," says King. "And if we don't help our mid-life women today, we're going to have an expensive problem on our hands 20 or 30 years from now."