WHILE THE NUMBER of babies born in this country dropped last year, these babies are likely to live longer than any previous year's crop. Both the birth and the death rates declined in 1986 -- along with, incidentally, both the marriage and the divorce rates. The National Center for Health Statistics has just published the numbers for last year, the most significant of which is the fertility rate. That's the number of babies born in relation to the number of women of childbearing age, and it was the lowest in American experience.

The life expectancy for children born last year was 74.9 years. That gives each of them an even chance of seeing the year 2061 -- and a bit more, since that life expectancy is based on 1986's death rates, which are likely to keep falling through the children's long lives.

But in most other developed countries, people live longer. According to a Census Bureau estimate, the longest lived are the Swiss at nearly 78 years -- no surprise, with all that fresh mountain air and those high incomes. But lives are very nearly as long in Hong Kong, which has neither. Japan also ranks almost as high, as do some of the Scandinavian countries. Life expectancy in Canada is more than two years longer than here, a remarkable difference between two countries whose societies are in many ways similar. Spaniards, Greeks and Italians, although less wealthy, also live significantly longer than Americans.

Infant mortality has been steadily declining in the United States, but, again, it's still much higher than in most other developed countries. That statistic reflects not only medical resources but a society's ability to organize itself to make the most of them -- an American failing. In 1986, of every thousand live births, 10.4 infants died. In Japan the current rate is 5.8 infant deathsper thousand, hardly more than half as many. Canada's rate is 7.5 infant deaths, and in Ireland -- a very poor country by American standards -- it's 7.7.

But it's the decline in fertility rates that casts the longest shadow forward. It will strongly influence not only Social Security taxes two decades from now but the whole breadth of national politics and culture. The very high fertility rates of the 1950s swung the country, in the late 1960s, into an infatuation with youthful music, clothes and interests. While those years, like the children, developed habits that required correction, they had much charm. It would be melancholy to think that the mood might slide to the other extreme two decades from now, turning Americans into a population gripped by theconcerns of the elderly -- medical care, stabil-ity and security. Fortunately, despite thetrends, births still outnumber deaths by the substantial ratio of seven to four in the United State