Middle-aged disco goers, brushing the thinning gray or dyed strands from their tinted bifocals, twirl slowly, (probably very slowly) tapping their well-shod toes, and sometimes canes, to the strains of Lawrence Welk rather than the Bee Gees.

The couples will drive from their regular night on the town in gas-saving, but luxury, cars to their in-town renovated rowhouses or their outer-Beltway farmhouses to greet their 0.68 children, or they'll be childless. They'll likely have no one but themselves on which to lavish their two-wage-earner, white-collar incomes and they probably won't mind.

This is one picture of the Washington resident of the 1980s who will experience more of some things -- money and consequently more places to spend it, working women, broken marriages, ethnic groups, singles and fun -- and less of others such as children and upward mobility of blacks. They'll be more ethnically diverse and less blue collar and, on the average, much older.

Washingtonians of the 1980s will start "acting more like independent adults and less like they're tied down by kids," said George Grier of The Grier Partnership research firm. They're getting out more at night, spending more on adult-oriented things than on kids."

A great deal of the new life style can be attributed to the continuing maturing of the post-war baby boom whose members so far have decided against a boom of their own. As the new decade unfolds, we find this influential group in the 30-to-34 age bracket and constituting 36 percent of the population.

By the end of the decade the protesters of Vietnam's bombing and trenches will be entrenched into a staid, establishment life style. The figurative bra burners of the 1960s will be burning more in their costly Cuisinart and microwave-laden kitchens or savoring the results of others in expensive restaurants.

In response to the new life style of the baby-boom bunch, household size has dropped nationally from 3.1 person per household in 1970 to 2.8 at the end of 1970s. In Washington, that decline has gone from 3.09 in 1970 to 2.76 projected for this year. This is expected to drop to 2.68 by 1990, according to a report released by the Metropolitan Washington Council of governments.

It will be the first time that households won't be large enough to accommodate two adults and a kid," Grier said, adding that although the size of the households will decrease, the number of households will increase.

Two reasons for the high rate of household growth are a "single boom and the baby boom come of age," according to a study of local trends by the Center for Municipal and Metropolitan Research. The singles boom results from proportionately fewer marriages, proportionately more divorces and separations, and a rising average age at which young adults marry for the first time. It is likely that the singles boom also reflects a higher rate of inmigration of single adults than of married adults."

The study continued: "In the 1960s the 15-to-24 age group increased by 48 percent in the U.S. population but by 83 percent in the Washington area." "Furthermore, the 5- to 14-year-olds increased by 44 percent in the Washington (area) in the 1960s in contrast to only 15 percent nationally . . . wThus the baby boom, which has had profound effects on American politics and on the American economy and American society, is magnified in the Washington metropolitan area," the study said.

Another trend from the 1970s will continue. "Incomes will continue to go up," Grier said. Because of the decrease in household size and increase in incomes, couples will have more money to spend.

As for those who already have reached middle age, they too may have more to spend. Grier predicts that they will reach the end of college tuition and room-and-board payments as their children age and leave home. "Most middle-aged people will get rid of that drain. That's a big, big stimulus to the area's economy."

The restaurant and theater boom is partly attributed to the change in life style as a result of the shift in household" size, Grier said. Discos and restaurants will continue at least for this decade, he added, but "maybe the discos will have to start playing more Lawrence Welk or softer music."

Grier pointed to the production by Levi-Strauss of Levis for men, jeans for the more mature-figured male, as an example of the continuing influence of the now-aging baby boom.

Local economists also predict that there will be proportionately fewer white Washingtonians and that the suburbs will become more integrated. There will also be a great deal of movement outside of the Beltway, particularly for those with children, and closer to the city for the two-wage-earner childless couples.

"There may be a particularly noticeable net outmigration of native whites from the (area) as a whole," the researchers said. "That is partly because immigrants -- especially Asians and Spanish-speaking people -- seem certain to loom larger in the population growth of the 1970s than in any previous decade in the history of the national capital."

That trend is expected to continue.

"It's transforming the area ethnically," Grier said. He added, however, that the number of blacks is expected to stay about 25 percent of the population, the same as it's been for at least the last 50 years, he said.

"There will be more diversity in the suburbs," Grier continued, "and a little less tendency toward segregation."

White collar occupations will expand as blue collar work is pushed farther out beyond the Beltway, Grier said.

"The proportionate expansion in higher-paying occupations seems most likely to continue through the 1980s," the research center study predicted. "The increasing labor force participation rate of women and the increasing rate of household formation seem more likely to moderate and possibly even stabilize or reverse in the course of the 1980s. It is at least possible that the upward mobility of blacks will also proceed at a slower pace in the 1980s."

Total population growth in the area has been dropping steadily during the last decade, and that trend is expected to continue, economists said. The source of growth in the area is almost entirely due to natural increases in the sizes of existing families rather than more families moving in, one study said.

The principal location of growth has become Northern Virginia and, partly because of high real estate prices, that growth extends far beyond the Beltway. According to the study, the rate of growth in Virginia and West Virginia has increased 10 times faster than that in Maryland.

The COG study adds that the District and Arlington, "both of which have experienced population declines through 1977, are expected to begin to reverse the trend by 1980 due to significant amounts of current construction." Growth in Alexandria is expected to be modest, the study said.

In Prince George's County, population growth is predicted to increase this year, while Montgomery County is expected to grow more slowly. "The sewer problem in Montgomery is expected to continue to affect growth until 1985," the COG study said.

Fairfax County, however, which was the fastest growing jurisdiction in the 1970s, will be "the most populous county in the region by the mid-1980s."

Spacious Loudoun and Prince William counties "have been experiencing rapid growth in the 1970s, and the intermediate forecasts show these trends continuing," the COG report said.