Fashion models over age 40 who once kept their gray at bay are rediscovering their roots--and capitalizing on a market with potential growth.

U.S. magazines such as Mirabella, Lear's and Moxie (based in Woodland Hills), that cater to mature audiences, are filling their pages with, "women who weren't born yesterday," as the Lear's promotional line reads. And, even traditional high fashion magazines such as Harper's Bazaar are devoting more space to seasoned models.

In the '60s and '70s, most magazine cover girls would retire well before age 30. Today, supermodels from those decades--among them model-turned-actress Beverly Johnson and Esme are enjoying revitalized careers. T. T. Zazzera, who heads the over-40 division at the Ford Agency in New York, says hers is the fastest-growing segment of the Manhattan agency. She has 30 models in the over-40 range.

"It's indicative of where the population is going," says Zazzera. "As baby boomers move into their 40s, advertisers are gearing marketing campaigns to them."

Among the leaders in this target advertising area, according to Zazzera, are shampoo manufacturers such as Jhirmack, Revlon and Clairol, who have developed formulas for mature, gray hair and prominently feature models with the same. At the other extreme, cosmetic companies are among the slowest advertisers to respond.

"They are still, for the most part, marketed to all ages. You're not seeing the over-age 40 woman advertising these products as much," says Zazzera.

One New York-based cosmetic company--Germaine Monteil--took a gamble on a silver-haired model in 1983 and the ad campaign that came of it lasted four years. The company conducted a "40 Is Fabulous" model search before hiring Tish Hooker as their spokeswoman.

"Women over 40 responded to it, and in many ways it helped change people's attitudes and it changed the business, too," a Revlon spokesman says.

Publisher Francis Lear says she has continually pushed for older models since she started her magazine in the fall, 1986, but admits it hasn't been easy persuading advertisers to use them.

"There are generations of women who feel alone in the world. They truly feel like outsiders when advertisers show anti-wrinkle creams on 17-year-old faces. It is insulting," says Lear, 66.

Lear has been urging advertisers to rethink their approach, but concedes she accepts ads that show young faces because, as she puts it, "one has to make a living."