ANN FARMER made her kids breakfast seven days a week. Six of those they got eggs. She'd bake brownies, pies and an occasional cake. Salt was always on the table, always on the food.

She and her husband, Harry, ate right along. They never had to watch their weight, or their diets. About 10 years ago--she was 44, he was 50--they began to gain weight, gradually. "It seems to get more difficult every year. It seems like we cut out more food and it's still more difficult." Now they watch their weight and their diets, cutting down on salt--he's prone to high blood pressure--sugar and fat.

Their kids have kids now, and they're doing things a little differently. They eat cereal--unsweetened, with no added sugar--for breakfast. Dessert means fruit--bananas, apples and raisins in the winter and anything ripe in the summer. Yogurt is always around; sometimes they eat it fruit-flavored, sometimes just plain with cut up banana. Much of the food that's served goes unsalted.

Chances are, says Dr. Simeon Margolis, a professor of medicine at Johns Hopkins, the change in diet will make a real difference to Ann Farmer's grandchildren. He'll concede that middle-aged people have health problems, that the incidence of obesity increases with age in this country. But he holds tight to the notion that people need to worry about their health long before they're 50 years old.

"When you're 20 you have 30 years to prevent the accumulation of cholesterol and other fibrous tissue in the arteries," he says. "If you wait until you're 50, your blood vessels are already narrow. If a 50-year-old is worrying about his cholesterol, then he should worry long and hard about his children's diet."

As a rule, he says, middle-aged people should exercise. They could stand to cut down on the amount of fat, sodium and salt they consume. He asserts that eating more fiber could be advantageous, and says that carbohydrate calories should come from high-fiber foods (whole grains, fruits and vegetables) rather than from sucrose (table sugar) and super-sweet foods.

"That need doesn't begin at 50," he said, about adopting healthful eating habits. "That's something that should be done an entire lifetime."

"This is the time when they get the first glimpse of nutritional problems that have been accruing over many years," says Dr. Myron Winick, director of the Institute of Human Nutrition at Columbia University, of 50-year-olds.

"Good nutrition is very important to establish in early childhood," he adds, for many reasons. First, fat children are much more likely to be fat adults. And fat adults run more health risks than thin ones.

Second, he says, atherosclerosis develops early in life. Your arteries don't harden in three years, it takes a lifetime for fatty deposits to accumulate.

Third, "children learn to like salt by watching adults, and by being exposed to salty foods." Some studies indicate that those who use a lot of salt in childhood are more inclined to high blood pressure as adults.

Good habits learned early may help prevent degenerative diseases that plague middle-aged people these days. But if someone gains weight gradually as he ages, says Margolis, he should not become over-anxious. "If you're talking about prevention, there's no evidence that a minor degree of overweight is in any way detrimental," he says, adding that, "We, as a nation, have long been sold a bill of goods about slight weight gain . Frankly, I think that is a lot of crap . . . There is too much emphasis on 'God, don't eat that, it's going to kill you.' "

Experts recommend a "reasonable diet" of the basic four food groups (meat, milk, fruits and vegetables and cereals) without too many extremes and, always, adequate exercise.