The median age of the voting population in the United States was reported yesterday as 40, down from 42 two years ago. The median age was 42 in 1970, not 1980.

The maturing of the post-World War II baby boom generation will boost the voting-age population to 169.4 million on Nov. 1, the highest in history, with the median age at 40, down from 42 two years ago.

This makes the number of potential voters 5 million more than in 1980 and nearly 29 million more than in 1972, the Census Bureau estimated yesterday. The increase since 1972 is partly due to the baby boom and partly because 11 million were made eligible when the voting age was dropped from 21 to 18 that year.

The voting-age population includes everyone 18 and over, regardless of whether they are citizens, but excluding about 550,000 federal military and civilian employes and their dependents overseas.

About 6 million of the 169.4 million total are legal aliens who will not be eligible to vote, and about 500,000 more will be ineligible because they will be in prisons, mental hospitals or other institutions, the Census Bureau said.

In 1980, 52.6 percent of the voting-age population voted for president, the smallest proportion since 1948, and 47.5 percent voted in House races. However, the percent voting in congressional races when there is no presidential contest is usually smaller.

In the 1978 off-year elections, only 34.5 percent of the voting-age population voted, and in 1974, the figure was 35.8 percent. Thus, the Census Bureau says, only about a third of the voting-age population is likely to vote in November.

The striking feature of the estimates is the youthfulness of potential voters. This year, 57.8 percent will be between 18 and 44, with about 20 percent under 25; about 26.3 percent aged 45 to 64; and 15.9 percent 65 or older.

And 52.4 percent will be women, who outnumber men 88.7 million to 80.7 million. This is the same proportion as in 1970 and is a function of age. Men slightly outnumber women between 18 and 24, but women outnumber men after 45 and make up 60 percent of the population 65 and older.

Although the new estimates for 1982 did not include regional and ethnic breakdowns, the bureau said that in 1980 the following ethnic and regional characteristics marked the voting-age population and presumably would be similar this year.

* The South, including the border states and Texas, has nearly one-third of all voting-age people (54 million), the Midwest about one-fourth (42 million), the Northeast 36 million and the West 31 million.

* Blacks constitute 10.5 percent of the voting-age population with 17 million people, but they are in greatest proportion in the South, 16.7 percent, followed by 8.9 percent in the Northeast, 8.1 percent in the Midwest and 4.9 percent in the West. New York has the most blacks of voting age, 1.6 million (12.4 percent), followed by California with 1.2 million (7.1 percent), Texas with 1.1 million (11.1 percent) and Illinois with 1.1 million (12.9 percent).

Proportionately, the District of Columbia has the most, 65.8 percent. States in the upper Midwest (Nebraska, the Dakotas, Iowa, Minnesota and Wisconsin), the mountain states (all but Nevada) and upper Northwest all have tiny proportions of blacks, 3.2 percent or less. For Maryland blacks constitute 20.8 percent; for Virginia 17.5 percent.

* Hispanics (including both whites and blacks who are also counted in the black minority total) constitute 5.5 percent of voting-age population with 9 million people. New Mexico had the biggest proportion, 33.1 percent.

However, nearly three-quarters of all voting-age people of Hispanic origin live in four states: California with 2.8 million (16 percent), Texas with 1.8 million (18 percent), New York with 1.1 million (8.3 percent), mostly Puerto Rican, and Florida with 629,000 (8.5 percent), mostly Cuban. For the District the proportion is 2.8 percent, for Maryland and Virginia 1.4 percent.

* There are also 864,665 American Indians, Eskimos and Aleuts, and 2.5 million Asians and Pacific Islanders.