FASHION : Suits for Mature Swimmers Fleshed Out : Those old favorites, skirts and the ‘40s-style ‘little boy’ leg, are back in style again.

About this time of year, a decision has to be made regarding beachwear. Should a self-determined, ‘90s woman with a lifestyle not primarily devoted to running track, appear at the beach in a suit small enough to conceal in a closed fist?

Would she be justified in waving this fist in the face of swimsuit designers? These are the folks, after all, who have conceived the string bikini, the thong and the cleverly disguised tank suit, which appears to offer bottom coverage until it is actually worn.

Contrary to popular belief, there are what we will call alternative swimsuits. Sports Illustrated editors may blanch at the thought (if they have the grace to blanch at anything), but out in swim land, there are suits with coverage that rivals the media reporting on Whitewater--with directly opposite results, that is: cover-up.

These garments may not be hanging in display windows, nor will they show up in every store. But with determination, the average swimmer can find a suit that allows her to cross the sand with ample spandex between her and the public eye.

What has happened, said Lynn Hulette of Lynn’s Swims in Thousand Oaks, is that some designers have at last realized that the boomers--who drive the marketplace--have aged.

“A lot of the manufacturers are accommodating people like they never have before,” she said. “This year, they really have brought down the sides a little bit and they’ve put more structure into the suits.

“I make it a point of getting those types of suits in, because ladies 25 and older really can’t wear those little skimpy nothing suits, and they don’t want to.”

Skirted suits are back in style, according to Hulette, as well as those with the “little boy” leg, a la the 1940s.

At Jantzen, there was more good news: A spokeswoman told us that their suits will come with a picture tag to indicate the fit of the suit without trying it on.

“You can judge how high the leg is cut, and from that you can get an idea of the coverage for the derriere,” she said.

So, they know there’s a problem. But, she said, Jantzen has carried a fuller-cut line all through the lean-swimsuit times.

Or, as she put it, “we have never forsaken our steadfast following of missy customers.”

Those of us who think that we are buying women’s sizes as opposed to juniors may be disappointed to know that we are thought of as “missys,” but maybe we’re over-sensitive.

Anyway, retailers’ reports are encouraging.

Rains in Ojai has the little-boy style as well as skirted suits, buyer Beverly Waid said. She says Elisabeth Stewart suits are reliable for a little extra fabric.

“We always bring in a few of them for the lady who is very conscious of her upper leg,” Waid said. “They do quite well.”

And at the Broadway in Ventura, a salesclerk recommended--among roomier brands--Jantzen, one style by Baja Blue and one by Mainstream, entitled--fittingly--the Hip Hider.

So, women who have advanced from juniors to, uh, missys might celebrate the expanding consciousness of an industry. Fuller suits could put confidence back in the beach stroll and return purpose to the age-old gesture of tugging a suit down upon emerging from a swim. This, we ask you to consider, is no small thing.

We did a field test at San Buenaventura State Beach to check on the season’s cover-up trend and found that area women may, in fact, be ahead of it.

Only about half of women beach-goers were in revealing suits such as minimal tanks and bikinis, none of which was the still-startling thong.

Many turned out in shorts, cutoffs and workout clothes; a few wore swim dresses or skirted suits.

One of those who appeared in a skirted style was Darla Hoback of Valencia, who said she had bought the suit several years ago.

“Of the women I know in my age group, 38 to 42, most of them have bathing suits like I am wearing,” she said.

Not yet aware of swimwear’s conservative swing, Hoback admitted that she had some misgivings about her outfit’s fashion appeal.

“I loaned this suit to my mother when she visited, and she loved it,” she said. “That made me think maybe I shouldn’t have bought this suit.”