Advances in combating heart disease, stroke and cancer have led to a significant decline in death rates for Americans of all age groups, except young adults, who are dying at greater rates because of auto accidents, homicide and suicide, the government reported yesterday.

Since 1976, death rates have increased among young men and women aged 15 to 24 because violent deaths now account for three-fourths of the fatalities of young people, up from half in 1950, according to the Department of Health and Human Services' annual report on the nation's health.

While car accidents are the leading killer among young white adults--accounting for 40 percent of all deaths in this group in 1979--homicides are most likely among young blacks.

While deaths among young blacks traditionally have been far higher than among young whites, the gap appears to be closing, the statistics show. This is because of a decreasing number of homicides and non-motor-vehicle accidents among blacks and an increase in car accidents among whites, the report said. Overall, the risk of dying is three to five times higher among young males of both races than among their female counterparts.

In general, the new report showed that Americans are taking better care of their health, with more quitting smoking and being screened for high blood pressure. A greater percentage of women seek crucial prenatal care early in their pregnancies, and their children are more likely to be immunized against childhood diseases, the government health experts said.

But despite gains in access to health care from the mid-1960s to 1980, blacks and other minorities still have lower life expectancy. And, says the report, the death rate for black babies is still nearly twice as high as for white infants.

"The facts do indeed show that American medical science is continuing its extraordinary progress in treating people after they get sick. But the other important message here is that the next era in health care must take us a step beyond traditional medical care--to stop illness before it strikes," said HHS Secretary Richard S. Schweiker.

In releasing the report, he announced plans for a new multi-media campaign--called "Stay Alive!" to alert the public about disease-prevention. The cooperative venture with the private sector is part of a department-wide health-promotion effort, which was also endorsed by the Carter administration.

The 1982 "Health, United States" report, compiled by the National Center for Health Statistics, also showed:

* After a rapid decline, U.S. birth rates have increased gradually since 1976. This trend is partly because women had postponed childbirth. The number of children each bears, however, is expected to decline.

* Overall life expectancy continued to increase through 1979, to 69.9 years for men and 77.6 years for women. But preliminary data for 1980 showed a slight overall decrease, thought to result from a flu epidemic that year.

* Declining death rates for heart disease and stroke helped account for the drop in mortality among adults 25 and older. Since 1950, mortality has fallen by about a third among those aged 25 to 44 and about one-fourth among groups over 44 years of age.

* Death rates for cancer declined over the past decade for those under 50, but increased for older people. However, the latest data suggests "a turning point" may also be occurring in the older age groups.

* From 1964 to 1980, annual physician visits increased from 3.9 to 5.5 per person for those in lower income groups, while stabilizing or even declining in higher income brackets. But, critics charge, this trend toward health care equality may be affected by more recent health budget cuts by the Reagan administration.

* The number of people being hospitalized continued to increase in the 1970s, especially among the elderly, but the average length of stays declined. Surgery performed in hospitals also showed changes, with fewer tonsillectomies among children, a decline in hysterectomies among women, cardiac catheterization--a diagnostic procedure for heart patients--increasing rapidly among men, and cataract removal continuing to increase among the elderly.

* The ratio of doctors to patients reached a record high of 20 per 10,000 population in 1980 and is expected to increase over the next decade.

* Health care expenditures continued to skyrocket, to $286.6 billion in 1981--an average of $1,225 per American, or 9.8 percent of the Gross National Product. Massachusetts led the country in per capita personal health and hospital care spending. The lowest spending for personal health was in South Carolina and for hospital care in Idaho.