"Oh, you're 29 years old," said the bridal consultant. "I can see why you're having trouble finding the right dress. It's always a problem for the mature bride."

The last thing a blushing first-time bride wants to hear is that she's over the hill, but that was the hint given at least one disgruntled Washington woman in quest of a grown-up wedding dress.

The "mature bride," however, is hardly alone. Of the more than 2 1/2 million women who jammed the aisles of America's churches and synagogues last year, one-fifth of those getting married for the first time were over age 25. And of these, one third were over 30.

Although they hardly need geriatric aids to get them down the aisle, these older brides are considering carefully how they, their fiance's and families can adjust wedding customs and traditions often geared to a bride of 18.

With the baby-boom generation now in their thirties and the median age for brides just over 22--and almost 25 for grooms--older and wiser couples are bending a few of the time-honored traditions. Although grand and more traditional weddings are back in vogue--the barefoot variety of a few years ago is on the decline--there are still many personalized variations on the theme.

Many brides shop for their wedding dresses and china not with their mothers, but with their fiance's. Some go all out with a long train and a dozen attendants. Others choose a well-cut Calvin Klein silk suit. More than one veil and headpiece have been selected to cover a few streaks of gray hair.

"This is a woman who has had experience in her life," says Barbara Tober, editor-in-chief of Bride's Magazine, a 50-year-old chronicler of American weddings. "She's probably been through several love affairs. Maybe she's lived with somebody. When it comes time to marry, she has definite ideas about what she wants."

In the June issue of Town and Country magazine, the editors did their own survey of weddings among what they termed "600 social couples." Among findings of their Newlywed Poll: large and grand '50s-style traditional weddings are back; engagement periods last an average of seven months and often include a period of living together; couples want larger families; one-fourth said "I do" after age 26.

For the upscale slice of society polled by Town and Country, the average wedding cost of those surveyed was $12,600, the average annual income of the couples to be married, $67,052. The magazine acknowledged that although the trappings of the ceremony hark back to the traditional, outlooks on what marriage will mean to a couple's relationship and careers are a 1980s phenomenon.

For example, says the magazine, "The 28-year-old vice president of an investment banking firm plans to hire a nanny to care for her two hoped-for offspring and is not planning to give up her $95,000 job."

For the couple still trying to figure out how they'll afford a honeymoon, that case may seem extraordinary. But the bottom line is this: Older couples are entering marriage with defined expectations and questions.

Doris Nixon, director of educational services for the National Bridal Service, a consulting agency, says she is "besieged" with questions from older brides who want advice on wedding finances, clothing and etiquette. Because society has changed so much in the past 20 years, she says, etiquette books have been unable to update fast enough to handle all the new problems.

"Once a couple is established in their own professions, their income is different and they end up often paying for their own weddings," says Nixon. "Yet the invitation is still often issued by the parents. In some weddings, the couple decides to share the wedding costs with both the bride and the groom's family."

"It's a party that celebrates the union of two families," says Tober. "Why shouldn't everyone contribute to pay for it?"

Wedding gifts are also different for couples who have each had their own homes and have two sets of virtually everything. These newlyweds don't want--nor need--toasters, irons and electric mixers. They already own them.

"They've had their apartment for a while, and their needs for household products are not as basic," says Gigi Warren, manager of bridal services at Woodward & Lothrop. "They want china, crystal and sterling, things to set an elegant table. And their taste is a little more sophisticated."

In a dress, "They are looking for something with a great deal of style that has to extend a bridal look, but is not the traditional, top-of-the-cake looking bridal dress," adds Warren.

Although Woodies and other area bridal salons do carry an increasing number of dresses and suits that fit that bill, the bridal industry is just beginning to catch on to the fact that many brides over age 20 don't want cascades of ruffles and 10-foot trains. They also can afford--and are willing to pay more--to get something they are happy with.

Psychologically, marriage can be an even more difficult step for an independent career woman. She is apt to resent relatives' assumptions that she will take her new husbands' name after the wedding or that she will immediately quit her jobs and begin raising a family.

There also may be mixed feelings about a grandiose ceremony.

"Some of these women feel slightly apologetic to society for not having gotten married yet," says Tober, "regardless of how strong and independent the woman is.

"Some of them even need to hear words of reassurance--somebody to give her permission to marry. Maybe it's an article she reads. Or maybe it's the sensitive bridal consultant who says, 'Yes you can wear white when you're 35 and of course you can carry a bouquet.' The important thing is that you look beautiful."