The young adults of the Pepsi generation are surging into Washington - fueling the real estate boom, bringing a new life to downtown, and, at the same time, contributing to the city's declining population.

Many of them are single or childless couples and they occupy houses once inhabited by larger families.

New population estimates released yesterday by the District of Columbia government show that the number of children and middle-aged adults has gone down substantially since the 1970 census. But the number of young adults between the ages of 25 and 34 has gone up 15 percent.

Overall, the city's population fell by 8.6 percent between 1970 and 1977, the new report said, and it dropped another 20,000 by mid-1978, according to U.S. Census Bureau estimates.

But James O. Gibson, assistant city administrator for planning and development said the population changes are "a sign of a resurgance not of a decline."

"We are attracting the young working age group who have higher skills and get higher paid jobs," Gibson said.

"The growth in town is within the age group where the skills are," Gibson said, "and that's an indication of the strength of the city."

Washington's Pepsi generation influx is part of a nationwide pattern, as individuals born in the baby boom after World War II come of age.

Big cities are getting a big share of young adults even though cities are losing other age groups said Marcia Kunen, chief of research for the city's planning office. "The big cities like Washington are where the jobs are."

The phenomenon involves both blacks and whites. According to the report, the number of white, young adults went up 25.4 percent from 1970 to 1974 as the city's overall white population fell by 17 percent. The number of black, young adults rose by 11 percent, the report said, even though the total black population fell by 5 percent.

"This is the walk-to-work, bike-to-work group," Gibson said, "and you can see their expansion eastward from the Dupont Circle area."

Gibson said more whites than blacks are involved in this movement, but that blacks with similar skills and education are taking part.

"These are the people with the kind of strength needed to survive in this city," Gibson said. "This is the revitalizing population."

Gibson said the data is not detailed enough to show how much of the increase in young adults has been caused by more of them moving into Washington and how much reflects fewer of them leaving the city once they come.

In the 1950s and 1960s, he said, Washington attracted large numbers of young people in their early 20s. But most of the whites, he said, left in their late 20s and 30s as they established families.

"I think more are moving in and more are staying," Gibson said. "Certainly, they're having fewer children so schools are less of an issue.... But right now we don't have enough [data] to be sure. We'll have to see the 1980 census."

The decline in school-age children - down 21 percent in seven years was caused by a sharp drop in births, which occurred nationwide, as well as by the move of black families to the suburbs, officials said. White families moved earlier, before 1970, the officials said.

"In many ways I think [the drop in children] is a plus," Gibson said. "It gives us breathing room in the schools and in the recreation centers. It gives us time to improve quality."

The new report, whose estimates are based on housing, birth, death and tax records, also shows that:

Washington's population decline from 1970 to 1976 was about average for big American cities, even though it was the steepest the District has ever had.

The proportion of blacks among D.C. residents was 75 percent in 1976, compared to 72 percent in 1970. However, it was down slightly from the peak of 77 percent black reached in 1975.

Besides young adults, the only age group in the city to increase since 1970 has been the elderly. The population over age 65 rose by 3.2 percent from 1970 to 1977, with the black elderly population increasing by 17 percent. The number of elderly whites dropped by 7 percent.

Even so, those over 65 still make up almost 22 percent of the city's white population - about double the national average, while elderly blacks make up just 7 percent of all blacks in the city,.

Some of the impact of these sociological factors can be seen in the estimates city reports give for individual census tracts.

For example, in tract 50, which covers the Logan Circle area, the number of whites increased from 600 to 1,000 from 1975 to 1977, while the number of blacks decreased from 5,700 to 5,200.

Earlier in the decade, from 1970 to 1975, the number of whites there had dropped sharply while the number of blacks increased.

In the 1960 census the area had over 3,200 residents - about 2,000 more than it does now. The proportion of whites - 20 percent - was about the same as in 1977.

Gibson said most of the whites who left the area before 1975 were low-income, while those moving in now are middle-class. Throughout the past two decades, he said, most of the blacks in the tract have been working class.

"There's a class factor in the revitalization of the city that's not strictly race," Gibson said. "It's a matter of class and education and skills."

Throughout the city, the proportion of blacks in different age groups - children, young adults, middle-aged and elderly - is generally about the same as the age structure of blacks in the entire nation.

The proportion of black young adults in D.C. though, is about one-fifth higher than nationally, and the proportion of children and elderly is slightly less.

The age distribution of Washington whites is radically different from the national pattern. The young adults make up 25 percent of all whites living in the city, compared to 15 percent nationwide. The proportion of children under age 14 is small, just 8 percent, compared to 22 percent nationwide. The proportion of elderly - 22 percent - is about twice the national average. CAPTION: Graph, The Changing City, By Alice Kresse - The Washington Post