The 1970s saw a rapid swelling of the elderly population in the Washington area, swift growth in the number of people living alone or in smaller, broken-family households and a rise in house or condominium ownership, according to an analysis of 1980 census data released yesterday by the Metropolitan Washington Council of Governments.

The COG report, which also found that minorities now account for one-third of area residents, details a dramatic shift in the makeup of the region that could have a major impact on local government planning and the demand--or lack of demand--for future services.

"The data are really a documentation of general trends we've known," said John McClain, coauthor of the report and COG's assistant director for planning and forecasting.

COG's 1980 census analysis, the second in an ongoing series of population reports, showed:

* The region--defined as Washington, immediately surrounding jurisdictions and the outer Virginia counties of Prince William and Loudoun--had fewer young people and more retirement-age residents in 1980 than in 1970.

The number of children under the age of 5 declined by 27 percent while the number of school-age children dropped by 16 percent compared with a decade ago. At the same time, the number of persons 65 or over increased in every Washington jurisdiction, registering a regional jump of 32 percent. Even the District of Columbia, which lost population overall during the 1970s, reported a 4 percent gain in the number of senior citizens.

In raw numbers, COG said, the region experienced a decline of 188,354 in the 18-and-under bracket. At the same time, the working-age population increased by 260,594 and the retirement-age population increased by 54,808. The total population rose from about 2.85 million to 2.98 million.

* 1970-79 was "the decade of the condominium."

Condominium units now comprise nearly 10 percent of the total housing units in the region, according to the COG analysis. Prior to 1970, there were only 976 condominiums in the Washington area. As of July 1, 1981, that number had increased to 94,989. During roughly the same 10-year period, total renter households increased by only 3.7 percent, from 485,523 to 503,615.

By 1980, 54 percent of the area's housing units were owner-occupied compared with 46 percent in 1970. The total number of owner-occupied households increased from 412,973 in 1970 to 587,777 in 1980, with gains in every jurisdiction in the region.

* There was an increase of 77 percent in the number of people who are divorced or separated, and a parallel 89 percent increase in the number of children living with only one parent.

The number of divorced or separated persons increased from 143,444 in 1970 to 254,808 in 1980, leaving almost one out of every four children under the age of 18 living with only one of their parents.

More than half of the region's one-parent families, 58 percent, were located outside what COG calls the "core jurisdictions" of the District of Columbia, Alexandria and Arlington. That was up from 45 percent in 1970. Over the decade, 81 percent of the increase in the divorced and separated population occurred outside the core area. Thirty-eight percent of separated and divorced persons lived outside the core in 1970; by 1980 it was 57 percent.

COG also reported that the region's household sizes were significantly smaller in 1980, with an average of 2.67 per household compared with 3.09 in 1970. This reflected what COG said were interrelated social and economic changes that occurred during the decade. These included the financial ability to purchase housing; the decision by many more women to enter the work force; the decision by many to marry later and have fewer children; and the large increases in the number of divorced, separated and one-parent families.

Additionally, COG reported that the region's population in 1980 included a higher proportion of blacks and other minority residents. During the decade, the region lost 108,756 whites while gaining 135,406 blacks and 100,566 of other races, chiefly from among Asian-Pacific groups. In 1970, the region was 74 percent white, 25 percent black and 1 percent "other"; by 1980, the proportions had become 67 percent white, 28 percent black and 5 percent "other."

As the decade concluded, the COG report noted, 52 percent of the region's minority population lived outside the District of Columbia, compared with 26 percent in 1970.