Washington — Winds of change blow around the old mom-and-pop American family.
Statistics in a new Census Bureau study reveal profound changes in some of America's basic relationships:
* More adults live alone than in 1970.
* Unmarried-couple households have more than tripled since 1970.
* America's marriage age is a little higher.
* Upward trend in divorce continues - it's ''a major marital status phenomenon.''
* Only 12 percent of children lived in one-parent families 10 years ago; now it's 20 percent.
The new study is a 64-page compilation of statistical tables with a six-page introduction. Some tables show little change in life styles; others are striking. For example, ''Increases in the tendency to postpone or forego first marriage and in the dissolution of marriage by divorce are two of the most important recent trends in the marital status of the population 15 years old and over.'' Median age for first marriage has risen since 1970 and is now 22.3 years for women and 24.8 for men. The introduction says this ''significant'' change ''reflects the tendency to postpone or to forego marriage, very likely in favor of continuing education and pursuing career opportunities.''
Meanwhile, some families are separating. ''The upward trend in divorce,'' the study says, ''continues as a major marital status phenomenon in American society.'' In 1981 there were 10,841,000 divorced persons and 99,792,000 married persons or a so-called ''divorce ratio'' of 109 to 1,000. This ratio was more than twice the 1970 ratio of 47.
''Women had a higher divorce ratio than men (129 vs. 88) reflecting the fact that men are more likely to remarry and do so more quickly after divorce than women.''
The report says that the number of unmarried-couple households more than tripled between 1970 and 1981. In 1970 half of these households were maintained by women, compared with just over one-third in 1981.
What happens to the children in separated families? A striking figure is the increase of children living in one-parent families; in 1970 it was 12 percent (children under 18 years old) now it is 20 percent. In numbers there were 12.6 million children living with one parent in 1981, or 54 percent more than in 1970 . This rise occurred even though the total number of children under 18 dropped from 69 million to 63 million in the same period. Most children lived with mothers (90 percent.)
The report analyzes data collected in the Census Bureau's March 1981 ''Current Population Survey.''
The new August study of ''Population Characteristics'' follows last month's on family ''Money Income and Poverty.'' This showed that family purchasing power declined in 1981 and that while the median family in America earned more it bought less. After adjustment for inflation, it reported the median family money income declined 3.5 percent between 1980 and 1981. This wasn't so bad as the 5.5 percent drop between 1979 and 1980, however. The study estimated the ''poverty threshold'' in 1981 for a family of four at $9,287, up from $8,414 in 1980. This meant, the survey concluded, that ''there was a 7.4 percent increase in the number of the nation's poor between 1980 and 1981.''
The Census Bureau is cautious in its descriptive analysis knowing that poverty is a red-hot issue in politics. It pointed out that its figures reflect only money income and exclude noncash benefits such as food stamps, public housing, and medicaid. It said of 1981:
''For the third consecutive year, families with no (wage) earners fared better relative to inflation than their working counterparts. The median income for families with no earners was $9,410, unchanged from 1980 in real terms. One-earner family income was $17,630, down 4.5 percent, while two-earner family income was $26,860, down 1.3 percent.''
How many people are poor in America? The report says last year's increase in poverty was distributed as follows:
* In 1981, there were 21.6 million whites and 9.2 million blacks below the poverty level compared with 20 million and 8.6 million, respectively, in 1980. There was no significant change among Spanish-Americans (3.7 million).
* Number of poor children under 18 rose from 11.5 million in 1980 to 12.3 million in 1981. The number of poor people 65 years old and over was unchanged at 3.9 million.
* About one-half of all families below the poverty level in 1981 were maintained by women with no husband present.