The Census Bureau said yesterday that 13 percent of the population, nearly one American in eight, lived below the poverty line last year, and that median family income after adjustment for inflation fell the most in any year since just after World War II.

The poverty number was up from 11.7 percent the year before. Among blacks, 32.5 percent were in poverty, and among Hispanics, 25.7 percent.

The agency said that in 1980 median U.S. family income, the income figure that half of all families were above and half below, was $21,023, a nominal increase of about 7 percent over 1979.

But it said that because of inflation the median family had 5.5 percent less purchasing power in 1980 than in 1979, "the largest decline recorded in the post-World War II era."

Commerce Secretary Malcolm Baldrige, in a statement, blamed the developments on "the combination of high inflation and last year's business recession." He said, "These adverse conditions harmed all Americans, but had especially harsh consequences for the working poor, who are among those most vulnerable to recession-induced unemployment."

The government poverty line, which is lifted each year to keep up with inflation, is used as an eligibility standard for many federal welfare programs. Last year it was $8,414 for an urban family of four, more for larger households, less for smaller, less for rural. There were 29.3 million below the cutoff.

For the aged, the portion below the poverty line rose to 15.7 percent, the second consecutive increase for the 65-and-over group. This is a reversal of the pattern that prevailed in the 1960s and 1970s, when the percentage of elderly people in poverty steadily fell, in large part because of increases in Social Security benefits.

The reversal comes at a time of possible retrenchment in Social Security. President Reagan has proposed benefit cuts, and Congress is preparing to act on them, perhaps next month.

"Cuts in Social Security, and particularly in the cost-of-living adjustment, will only cause more dramatic poverty-rate increases in the future," said Laurie Fiori, lobbyist for the American Association of Retired Persons.

For the past generation, the proportion of people living in poverty has generally been going down rather than up. Census figures show that in 1959 the poverty ratio was 22.4 percent for the population as a whole and 35.2 percent for those 65 and over.

Since then, poverty has declined steadily, with occasional slight blips upward to reflect a bad year for the economy. The year with the lowest poverty ratio was was 1973, 11.1 percent for the overall population.

Since then it has crept up and down slightly. In 1979 it rose a tiny bit over the previous year, and for the population as a whole it was 11.7 percent; for the aged, 15.2 percent.

The poverty definition has been a subject of debate in recent years. In determining whether a family is above or below the line, the Census Bureau counts only cash income, which includes welfare and Social Security payments. It doesn't count "in-kind" income such as food stamps, housing assistance, Medicare or Medicaid, which supplement the cash income of the poor substantially and cost the government billions annually.

For that reason, many people, such as presidential assistant Martin Anderson and Congressional Budget Office experts, have argued that the way the Census Bureau measures poverty is misleading.

If "in-kind" income were counted, according to the CBO, the proportion of the population living in poverty would only be about half the Census Bureau figures.