It sometimes seems that modern art is the province of the young. Old age often brings with it a hardening of the vision, a rejection of the new. Painter Mary Cassatt, who had once fought for the moderns, became, as she neared 80, bitterly intolerant of all styles of painting newer than her own.

Adelyn D. Breeskin's father was similarly limited. She recalls that "he loved art, but only older art. He couldn't see the moderns." Nothing of the sort afflicts his daughter.Her mind has never closed. Though she is in her 80s now, Adelyn D. Breeskin's eye seems permanently young.

As a curator and scholar, Breeskin has been working with the new, and with the nearly new, for more than half a century, and is at it still.

She was the first woman to be named director of a major American museum, the Baltimore Museum of Art. She is a renowned Cassatt specialist, having written two books on that Impressionist from Philadelphia, and both are standard texts. Her most recent title (she's had many) is "Consultant for 20th-Century Painting and Sculpture" at the Smithsonian Institution's National Collection of Fine Arts. "She is supposed to be some kind of curator emeritus," says her colleague, Peter Bermingham. "But she is the least emeritus emeritus that I ever saw."

Two quite different shows of older modern art are concurrently on view at the National Collection, 8th and G Streets NW. One, of Cassatt's superb pastels and color prints, is eye-delighting, beautiful. The other, given to the less distinguished paintings of Attilio Salemme, the minor New York surrealist, is disconcerting, quirky. The Cassatt exhibit is a study of a master. The Salemme seems the sort of footnote exhibition regularly mounted by NCPA. These shows have only one thing in common: For different reasons, Adelyn D. Breeskin organized them both.

She is sitting in her office at the National Collection managing to seem at once serene and busy. Breeskin has about her an air of unflusterable grace. She is tall, white-haired, well spoken, unfailingly polite.

Though most museum curators are diplomats of sorts, a set of special pressures act on those, like Breeskin, who deal with contemporary art. Because the work they show is often new or unfamiliar, their decisions are not based on historical consensus. Because commissions sales and reputations often are affected by the museum shows they organize, they are often second-guessed.

Breeskin has dealt, and deals still, with imploring artists of all sorts, with promoter, dealers, students, and with the demands of collectors and trustees. She keeps her cool. In a field full of backbiting, she seems to have no enemies. To the wealthy and important Breeskin does not fawn; to the young and inexperienced she does not condescend. She is an independent woman. One does not always sense that in her presence, so polished are her manners, but one sees it in her shows. A Brush With Bryn Mawr

She was born Adelyn Dohme in Baltimore in 1896. Her father was a chemist and a founder of the pharmaceutical firm that was to become Merck Sharp & Dohme. Her family was prosperous, Victorian. She enrolled in Bryn Mawr College at the start of World War I. Like other proper young ladies, Breeskin was expected to graduate, then marry. But, instead, she looked around the Bryn Mawr campus, disliked what she saw, and set out on her own.

"At first I had decided that I would be an artist. I am glad I did some painting. It is good to know the feel, the touch of brush on canvas. I was going to be a mural painter, perhaps because I'm so tall."

Form Bryn Mawr she moved to Boston, where she entered art school and enrolled in courses at Radcliffe, Harvard and Boston University. "I never graduated. I didn't think of needing to graduate. I knew exactly what courses I wanted to take, and I took them," Breeskin says.

Breeskin took her first museum job - in the print rooms of the Metropolitan Museum - 60 years age. "I worked for William M. Ivins Jr., a wonderful man. I was devoted to him. He was, incidentally, one of the first scholdars to closely study Mary Cassatt's prints."

In 1920, she left the Metropolitan to marry Elias Breeskin, a musician of whom her family did not entirely approve. That didn't stop her. Divorce was seen in those days as something of a scandal but that did not stop her either. When her marriage dissolved in 1930, Breeskin, now a single parent and the mother of three daughters, returned to Baltimore and took a job organizing a new department of prints and drawings at the Baltimore Museum of Art. The Five-Year Act

She began rising through the ranks, but her progress was not swift. For though her skills were soon apparent, Breeskin was a woman were, in those days, almost never given top museum jobs. She because the Baltimore's general curator in 1938. She was named acting director in 1942.

"The war was the only reason I was given the promotion," says Breeskin. "The men had gone away so the trustees turned to me. Even so they hesitated. The named me 'acting' difore they gave me the full title - fore they gave me the ful title - which I kept for 15 years. When it comes to directing large are museums, women don't have a much better chance even today.I think that is sad. One reason they are still excluded is that museum boards are businessmen are apt to be conservative and conventional. Many of them still believe a woman's place is in the home."

While in Baltimore, Breeskin gave Milton Avery his first museum show. "It was devoted to him," she says. "He never talked. He let his wife, Sally, do all the talking."

It was in Baltimore, 42 years old, that Breeskin gave Mary Cassatt's graphics their first museum show. Cassatts on Sale

"Cassatt was far less famous then than she is today," says Breeskin. "Because I thought her graphics splended, and hoped to get them out, I persuaded the museum to let me offer them for sale. The prints cost less than $100 then, the pastels about $250. Nowadays a good Cassatt pastel might bring $200,000. In 1936, in Baltimore, not a single picture sold."

Shifts in popular taste, and particularly the growth of the woman's movement, have increased the attention paid Cassatt. Feminist historians regard her, rightly, as a star. Once she was regarded merely a as a follower of her friend Edgar Degas, but that is no longer the case. In 1970, she was given a major retrospective at the National Gallery of Art. Earlier this year Cassatt was the subject of a special program on national TV. Breeskin might have done a show of Cassatt paintings that rode the Cassatt wave, but that was not her motive. Instead she decided to mount a small exhibit that stresses the artist's innovations.

When Cassatt drew faces, she moistened her pastels and worked the goo like oil. The resulting likenesses - primarily mothers and children - are finely modeled, sharply focused. But in the zones behind her figures she used her crayons with abandon. Her almost abstract backgrounds have something of the wildness, the freedom, of post-war painting.

While many Cassatt oils recall the towering presence of Degas, Cassatt's color prints are entirely her own. Their striking compositions, unexpected colors, and volume-rounding outlines somehow blend the spirit of French art with that of Japan. Tweet, Chatter, Hum

While the Cassatt show is a treat, Breeskin's Salemme exhibition is a curiosity, one of the Collection's efforts to display the unfamilar footnotes of art history. Attilio Salemme (1911-1955) made odd works that appear to link dream-inspired surrealism with late hard-edge art.

The peculiar stick-like figures in a typical Salemme - one picture by Salemme looks much like another - resemble squared-off, costumed robots. As they lined up for the viewer, like actors on a stage, one can almost hear them tweet, chatter, hum. Emptiness surrounds them. "His paintings," says Breeskin, "remind me more of the writings of Franz Kafka than of any other painter I can name."

Because Breeskin has done so much research on the paintings of Mary Cassatt, there are those who feel those two aristocratic women must have much in common. NCFA director Joshua Taylor disagrees. "I think Adelyn and Cassatt would have liked each other," says Taylor. Cassatt was in her way thoroughly conventional especially in later life. No one who knows her well would say that about Adelyn." Taylor and Breeskin are in the same carpool. She drives him to work.

Though an older venerated figure in a tradition-ridden field, Breenskin has never let conventional expectations determine her behavior. In the spring of 1972, the first issue of Ms. Magazine' ran a two-page ad underneath the headline, "We Have Had ABortions." Billy Jean King and Anais Nin were among the 53 women signing. So was Breeskin.

"Were I younger," said Breeskin, "I'd love to go to the moon. I really would. I'd go.