"CHILDHOOD," "Youth," "Manhood" and "Old Age," those four moralizing landscapes, are among the finest Thomas Coles in the National Gallery of Art. They have been there for a dozen years. Now, they've subtly changed.

The canvases have not been touched, their sermons are the same. Yet somehow they look different. It is their gilded frames.

"The Voyage of Life," Cole's half-religious vision of the travels of the soul, is unabashedly romantic. So, too, are the gleaming frames, lush with golden grapes. At last, they have been reunited with Cole's paintings.

That reunion tells us something about the shifting standards of American museology. It required time and kindness. And two moves of switch-and-swap.

Cole produced these paintings--and probably designed the frames that now surround them--in the early 1840s. When new, they achieved fame, but then they disappeared from sight.

We now know where they went. They were purchased from the artist in 1845 by one George K. Schoenberger, who installed them in the gallery of his Cincinnati mansion. By 1908, his house--now owned by the Bethesda Hospital and Deaconness Association--had become an old age home, and Schoenberger's small gallery had been turned into a chapel. The pictures were discovered there in 1962. After long negotiations, they were purchased by the National Gallery for six figures. The gallery, however, chose not to buy the frames.

There were, at the time, two quite different reasons for the gallery's decision. The first was diplomatic. The second was a matter of then-prevailing taste.

Rather than strip its chapel, the old-age home decided to replace the missing pictures with photographic replicas. Color photographs were taken (and coated with thick varnish) and then slipped into the antique frames on the chapel's walls. New frames, also gilded, but considerably less ornate, were acquired to replace the originals the gallery had left in Cincinnati. "The frames we have decided on," one curator observed in 1972, "are better."

At least newer. They had not been scratched or chipped. But despite their fine condition, the Gallery's new frames looked peculiarly wrong. Their style was Park Avenue Federal. They were classically austere.

John Wilmerding, then the gallery's curator of American art, now its deputy director, felt himself annoyed by those narrow, stripped-down frames every time he saw them. "They didn't grace the paintings, they clashed with them," says Wilmerding. "They made me thing of someone wearing studs, black tie--and tennis shoes.

"Only in the past 10 years, perhaps in the past five, have scholars recognized the visual importance of authentic period frames," says Wilmerding. "Cole's frames are theatrical. Those grapes and gilded leaves that jut into the room are much like the proscenium. They are there to help the viewer enter another world."

In 1981, Wilmerding went to see the old frames in the Cincinnati chapel. "When I saw them my heart sank. Somebody, probably the house carpenter, had covered them completely with gold radiator paint. But I felt they were so clearly right, we ought to go ahead."

Would the old-age home, he wondered, contemplate a trade? The gallery would do the work; it would replace the old frames, the ones around the photographs, with the new frames it had purchased--new frames, he observed, that were covered with gold leaf, not with radiator paint. A small honorarium, "a sweetener," says Wilmerding, also was made available. The old-age home agreed.

It took almost a year for the gallery's conservators to strip the radiator paint, make the molds required to fix the damaged grapes, and restore the antique gilding. But now the work is done.

No museum in America gives more consideration to its installations. Wilmerding believes that it does not make sense to worry about labels, wall colors and lighting while paying no attention to out-of-period frames. At Wilmerding's direction, many of the gallery's familiar American pictures--by Bellows, Twachtman, Peto, Church and Childe Hassam--have recently been given gilded antique frames. "If you are going to put all your marbles on originals," says Wilmerding, "why not go all the way?"