The U.S. population is likely to rise by about a third to 309 million in the year 2050, then begin to decline, the Census Bureau said yesterday.

It was the first time the bureau had classified zero population growth as a likely outcome in this country.

The bureau also said 21.7 percent of the population is likely to be over age 65 by 2050, up from 11.4 percent now, while blacks would probably make up 16.8 percent then, as against 11.9 percent now.

The new report said female life expectancy is expected to rise to 83.6 years in 2050 from 78.3 now and male life expectancy to 75.1 years from 70.7.

The U.S. population is now about 232 million and is growing at a rate of about nine-tenths of 1 percent a year.

The bureau said this growth rate is likely gradually to fade and the nation is likely to reach zero population growth by the middle of the next century.

This projection is based partly on projected continued low fertility rates for women. They are expected to average only about 1.9 births each in the next century, so low a rate that the population would decline were it not for immigration and increased life expectancies.

The bureau is assuming, among many other things, that the percentage of women in the labor force will continue to rise.

In the past the bureau has often been wrong about fertility swings and population trends. It failed to predict both the baby boom after World War II and the baby-bust of the 1970s.

The assumptions in yesterday's report are part of the bureau's intermediate population projections. There are higher- and lower-growth projections as well, but they are regarded as less likely. The inter-mediate vision of a gradually aging population with a continuing low birth rate assumes immigration will remain constant at about 450,000 persons a year.

The bureau made these other projections:

* The median age of the population will rise from the current 30.3 years to 41.6 years in 2050.

* The ratio of people of working age (ages 18 to 64) to people of retirement age (65 and over) will drop from 5.4 to 1, where it is now, to 2.6 to 1 by 2050.

* People 85 years and older, who now make up 1 percent of the population, will make up 5.2 percent in 2050.

These expected alterations in the age structure have already been noted by social scientists and are expected to have serious consequences for social policy and government programs, as well as for patterns of consumption and the economy generally.

Large increases in the proportion of extremely old people are expected to increase the need for nursing homes, for example, and the general aging of the population will further drain the Social Security and Medicare systems. All this will happen as the proportion of working-age people which can be dunned to support the elderly is declining.

* The number of births per year, now 3.6 million, will rise slightly to 3.9 million in 1988 as the post-World War II baby-boom generation has babies of its own. But then it will start declining until it hits 3.5 million annually in 2050 and never again will it reach the 4 million level attained during the height of the baby boom.

As a result, starting in 2035 the numbers of deaths will exceed the number of births, and only immigration will keep the population growing until 2050.

The bureau said different developments in fertility could radically alter the picture shown by the intermediate scenario. For example, if fertility turned lower and women of child-bearing age had only 1.6 babies each, the population in 2050 would be 257 million. On the other hand, if fertility were 2.3 births per woman, population would be 379 million by mid-century.