Levi's (and others) apply the stitch-in-time theorem

The sounds of a ball slamming into a wall and of heavy breathing tell you the scene is an athletic club. Then a confident male voice in the radio advertisement explains how his new Levi's for Men thoughtfully provide the extra bit of room his older body requires.

The baby boom is approaching middle age. And manufacturers are chasing after it, changing advertisements or products or both in a bid to keep selling to the 76 million people born between 1946 and 1964.

''Marketers are following the demographics,'' says Mort Galper, associate professor of marketing at Babson College in Wellesley, Mass. ''If that means changing the product or changing the way the ad is put together, they are doing it.''

Perhaps no company has a greater interest in holding onto baby-boom customers than Levi Strauss & Co., the San Francisco-based manufacturer of blue jeans, the ''Pepsi generation's'' unofficial uniform.

''Our supreme object is to make sure the postwar baby-boom people maintain a favorable attitude about jeans,'' says James McCarty, national advertising manger for Levi's Jeanswear Division.

Normally, men start buying slacks instead of jeans between ages 30 and 35. ''Slacks just fit looser than jeans,'' Mr. McCarty explains. To keep these key consumers in jeans as they age, Levi's for Men have been made roomier in the waist and thigh.

While Levi has changed both product and advertising, other firms are adjusting only their advertising. ''There is less use of rock and roll, and blue jean types are less numerous in the ads,'' notes Martin R. Warshaw, marketing department chairman at the University of Michigan.

Numerous advertisements show manufacturers catering to an aging baby boom. For example:

* Skippy peanut butter's story is being spread by Annette Funicello, the favorite Mouseketeer of many baby boomers.

* Johnson's baby powder is being pitched to adults by New York Yankee hurler ''Goose'' Gossage.

* Noxema skin cream ads feature ''Mork and Mindy'' star Pam Dawber advising: ''As you get into your 20s you have to watch out for dry skin.''

* Vitamin-supplement ads play down the product's market among senior citizens and instead stress the line ''you are not too young. . . .''

For marketers, the baby boomers' appeal is the dollars they have to spend. In 1980 the mean annual income of a 25- to 34-year-old was $13,201, up from $6,828 ten years earlier. And Time magazine estimates the group's total annual income is $424 billion.

Despite the baby-boom generation's wealth, there are still risks in wooing it. If a firm uses its name on products too different from its main line, the value of the brand name ''may become diluted,'' says John Diefenbach, president of the Landor Associates, a marketing consulting firm.

Another risk is that to attract Pepsi-generation members, firms may have to use ''appeals that are not attractive to other market segments,'' Mr. Warshaw notes. The danger is that the appeal to baby-boom members will make older buyers say ''they obviously do not want me.''

A more serious problem is the possibility a new marketing approach will cost a firm customers in the youth market where its original strength lies. ''To appeal to a new segment, you have to talk to certain attributes and feelings which differ from those in the orginal marketplace,'' Mr. Galper says. ''There is a risk of losing the appeal to the younger. They may be turned off that it is something for Dad.''

This risk is especally great when the product is changed as it has been in Levi's for Men. In addition to providing extra room, Levi's for Men are a blend of denim with polyester while the original jean is just denim.

The compostion of the jeans was changed becuase ''females have a heavy degree of influence on men 25 to 49. (Men) go shopping with their wives or their wives do the shopping.'' Mr. McCarry says. Thus Levi Strauss wanted a product that does not require ironing.

Still, the jeansmaker contends offering fuller-cut, permanent-press jeans for men will not hurt sales to younger buyers. Nevertheless, Levi Strauss is making sure its $40 million line of jeans for men does not cut into the billions of dollars of regular jeans it sells.

''Through purchases of (advertising time) you can alleviate some of the problems connected with . . . cross imagery,'' Mr. McCarty says. For example, Levi's for Men are advertised on radio programs emphasizing news, talk, or sports. Younger customers are reached on radio programs where music predominates.

While reaching aging members of the baby-boom generation has its risks, ''this is going to be a mainstream event '' in marketing, Professor Galper predicts, ''as marketers become more aware of the the impact of demographic changes on their own business.''