"Trades coming in are worn out pieces of iron," laments one Washington car dealer.

Thrifty Americans are holding onto their older cars longer. High prices, high interest rates and a multitude of demands on family budgets are forcing this "throw-away" society to cool off its love affair with shiny, new cars.

People can be seen all over town holding their heads high in well-seasoned vehicles. Success may no longer be linked to a new car in the driveway, but to knowledge of the best service station.

The average age of cars in use -- 6.6 years -- in 1980 was the highest since 1953, according to the Motor Vehicle Manufacturers Association of the U.S., Inc. Sales of used cars by franchised dealers were up 9 percent for the first 10 months of 1981, according to the National Automobile Dealers Association. Domestic new-car sales for 1981 were at the lowest level since 1961, dropping 5.65 percent from the previous year, and for Jan. 10-20, 1982, were down 15.5 percent from last year.

"We sell three used cars to every new car, and last month it was almost four to one," says Les Eldridge, manager of Woodbridge (Va.) Lincoln Mercury Corp.

"A new '82 Cougar XR7 sells for $12,000, while an '80 Cougar XR7 with 20,000 miles costs only $6,000, or half the price of the new model. The '80 cost $8,000 new. This can mean a savings of $150 to $200 a month when you consider interest costs.

"It's cost-efficient to buy the used car, and even many people who could afford a new car, buy the older one because it's smart. The status of having a new car goes out the window when the buyer considers the difference in costs."

Advantages of a new car can include, of course, lower maintenance expenses, longer use, better fuel economy and a higher price at trade-in, but $6,000 savings up front is hard for consumers to ignore: $6,000 will buy about 4,285 gallons of fuel, or something like four years' worth for a car averaging 15 m.p.g.

Another factor Eldridge says contributes to higher used-car sales is the availability of warranties. A two-year warranty for a 2-year-old car costs about $300.

Because late-model used cars are in demand, more of their original purchase price is being returned when they are traded in. And surprisingly many used-car dealers report that large American cars -- scorned a few years ago for their gas inefficiency -- are now sought after.

Many dealers report that buyers seem less concerned about EPA ratings because newer cars offer competitive fuel economy. However, David Baird, used-car manager for Stohlman Volkswagen and Subaru, Inc., for one, says his customers still consider fuel economy a top priority. With the bulk of his business in high-mileage used cars, Baird says his sales volume was up 15 percent last year.

Some dealers believe it was the shortage of gasoline rather than its expense that pushed consumers into smaller cars. Now that gasoline supplies and prices seem fairly stabilized, car buyers are considering other factors, such as comfort, safety and convenience.

"From midsize up is our biggest business, and 1- to 2-year-old cars are the biggest sellers," says Eldridge. "People are tired of small cars with little room or comfort. And they are rediscovering that American automobiles are good-quality cars offering excellent mileage."

Says John Koons Jr. of JKJ Chevrolet: "Larger cars are by far the most popular on the used-car lot. Even on new cars, consumers are discovering that per pound, a larger car is a much better buy."

Moore Cadillac used-car manager Thomas O'Brien notes that his company's used-car sales last year were up 15 percent over the previous year.

"A lot of people are first-time Cadillac owners," says O'Brien. "Others are senior citizens who previously bought a new Cadillac and now turn to a year- or 2-year old model because they want the quality of the car but cannot afford the newest models." (A 1982 Cadillac Fleetwood costs $21,000; used, with around 20,000 miles, about 14,000, including a one-year warranty.)

Says Ray Smith, Euro Motorcars used-car manager, about the classically resaleable Mercedes-Benz: "The most desirable cars right now are '77s and '78s selling for around $10,000 or $12,000, but I have none." (New models are $20,000 to $50,000.)

Despite all of this, a number of dealers say they believe the auto-loan interest rate will come down and anticipate -- they say with pent-up hope in their voices -- a stampede of pent-up demand for new cars.

But meanwhile, if you're hanging on to your old car or buying a used one, you're not only thrifty, you're downright trendy.