For the first time since 1950, there are as many Americans above the age of 30 as below it, according to 1980 census figures released yesterday.

The median age has risen by two years in the last decade and will rise more sharply over the next three decades, according to Census Bureau official Don Starsinic. The figures underline the dilemma the Reagan administration and Congress are facing -- how a shrinking population of working age people can provide the economic well-being of a mushrooming elderly population.

The report also in cluded new racial and ethnic population figures by region, indicating, for instance, that the number of Asians and Pacific Islanders has more than doubled, with the heaviest concentrations in California and Hawaii. That includes Vietnamese refugees who fled here as the war ended in the mid-70s.

Age statistics show the number of people 65 and older has grown over 20 percent since 1970, to 25.5 million, or 11.3 percent of the population. Forty years from now, that number is expected to almost double to 45 million.

Less than one-fourth (22.6 percent) of the American population is under 15.

Florida, a popular retirement haven has the highest median age of any state -- 34.7 years. Utah has the youngest, with a median age of 24.2, largely because it has maintained the highest birth rate of the states, officials said.

The District of Columbia population has a median age of 31.1, older than the national median. Eighteen other states concentrated in the Northeast have older, including New Jersey (32.2 years median age), Pennsylvania (32.1) and New York (31.9). Maryland is barely above (30.3) and Virginia barely below (29.8).

Other states with below median-age populations include Alaska (26.1), Wyoming (27.1), New Mexico and Louisiana (27.4).

Among racial groups, whites are considerably older than Hispanics and blacks, the figures show.Only 21.3 percent of whites are under 15 years of age, and 12.2 percent are 65 or over. That contrasts with 32 percent of Hispanics and 28.7 percent of blacks under 15. Only 4.9 percent of Hispanics and 7.9 percent of blacks are over 65.

"Hispanics and blacks traditionally have had higher birth rates than the white population," Starsinic said. "But the rate for all of them is declining."

The nation has been aging steadily at least since the turn of the century, when the median age was about 23, Starsinic said. By 1940, it was 29. After it peaked at age 30 in 1950, the enormous number of births after World War II caused a two-decade youth boom when the median age dipped to 29.5 in 1960 and to 28 in 1970. Now the long-term trend has resumed.

Declining birth rates combined with aging of the enormous baby boom population and the tendency of Americans to live longer, have driven up the median age, the census report said. The age distribution by states reflects, among other things, migration patterns.

The ethnic and racial figures indicated that about 53 percent of the nation's 26 million blacks still live in the South. After decades of migration to the North and West, earlier census surveys have showed that since 1970 almost as many blacks moved back to the South as moved from South to North.

In seven southern states, more than one-fifth of the population is black: Mississippi (35.3 percent), South Carolina (30.4), Louisiana (29.4), Georgia (26.8), Alabama (25.6), Maryland (22.7) and North Carolina 22.4 (percent).

In the District of Columbia, 70.3 percent are black.

Some 14.6 million, or 6.4 percent of the U.S. population, identified themselves as of Spanish origin. More than 60 percent live in California, Texas and New York.

They made up at least 10 percent of the population in five western states, including New Mexico, Arizona and Colorado.

Census figures reported earlier had shown significant increases in blacks and some ethnic groups, but officials attributed the differences in part to changes in counting methods and also in the way people identify themselves on census forms.