The nation's black population grew twice as fast as the white population from 1980 to 1984 and reached 28.6 million last July 1, according to the Census Bureau's latest estimates.

The bureau said blacks constituted 11.8 percent of the population in 1980 and make up 12.1 percent now -- by far the largest minority group in the country. The Hispanic-origin population, which consists of blacks and whites and is not counted as a separate racial group, totaled about 15.4 million in April 1982, according to other Census reports.

The total U.S. population July 1 was 236.7 million, including armed forces personnel overseas.

The bureau said a moderately higher fertility rate among blacks than among whites caused the black population to grow 1.8 million, or 6.7 percent, from 1980 to 1984. In contrast, the white population rose 3.2 percent.

Although the median age of both blacks and whites increased about 1 1/2 years from 1980 to 1984 because of the aging of the baby-boom generation, blacks constitute a far younger group. The black median age in 1984 was 26.3 years (meaning half were younger and half were older), while the white median age was 32.2 years.

The bureau said the black median has increased every year since 1967, when it was 22.1 years, and the trend is expected to continue.

Black females outnumbered black males in 1984 by 1.4 million -- about 15 million to 13.6 million. The disparity was greatest among older people because black women, like white women, live longer than their male counterparts.

Among blacks, the fastest-growing age group in the 1980-1984 period was people 85 and older. This group increased by 27 percent. Longevity increases due to medical and nutritional advances also have been noted among whites, and the proportion of the "old old" Americans is expected to increase markedly in the future.