Boston — Three cheers for demographics. Population trends should make life for the nation's economic policymakers a lot easier in the 1980s. In fact, investment manager Roy C. McKay says, ''They could create a golden decade - one not without its problems, but it should be easier for handling inflation, and include a population with more disposable income.'' More Americans should be prosperous.
The most important population factor is the decline in the number of people entering the work force. In the last 12 months some 1.5 million new workers sought jobs. In 1978 the number of new workers was about 4 million.
If that same 4 million had sought work in the last year, the unemployment rate would likely be 11.5 percent instead of November's 8.4 percent, reckons Mr. McKay, president of Gregg, McKay, Knight & McKay Inc. of Avon, Conn., an investment firm managing some $45 million.
He continues: ''If unemployment were 11 percent today, there is no question it would be Public Enemy No. 1. As it is, we can continue to try to control government spending in order to fight inflation.''
In other words, the slower entry of new workers into the economy in the next several years should enable the Reagan administration and its immediate successors to concentrate their efforts on trimming inflation. The so-called economic ''discomfort index,'' which is merely the addition of the inflation and unemployment rates and which was running in the 20s during much of last year, could fall into the single digit area, Mr. McKay predicts.
This decline in the new entries into the work force arises primarily from the aging of the baby boom generation. Its members are now 25 years and older and already established in the work force. There are far fewer people in the generation just now finishing school. Mr. McKay suspects there could indeed develop widespread labor shortages in a couple of years.
Most of the baby boom children now are married, and many are having children. The nation is undergoing a baby boomlet. Nonetheless, the so-called ''dependency ratio'' has dramatically improved. In 1965 there were 15 dependents for every 10 workers. By 1980 there were fewer than 10 dependents for every 10 workers.
This has several important economic implications:
* There will be many more affluent families in the nation.
Barbara J. Muller, an associate economist with A. Gary Shilling & Co., economic consultants, calculates that the percentage of total households earning real gross incomes of $22,900 (in 1980 dollars) and over should rise from roughly one-third currently to a record 45 percent in 1990.
That's because the baby boom group is moving into higher income job levels. In many cases, both husbands and wives work. In general they are better educated than members of earlier generations and thus tend to earn more. Moreover, the Reagan administration's moves to trim the tax burden for those in higher tax brackets will benefit them. Altogether, there should be another 12 million households in this ''upper income'' category, she figures.
* The nation should be saving more of its income.
Affluent families save more than poor families. Many of the baby boom families have already bought their homes, cars, refrigerators and other expensive household goods. This will be more true by 1983-84. They have relatively few children, who are always expensive. They can look at setting aside money in individual retirement accounts (IRAs) or other investments.
Mr. McKay notes that some 45 million Americans are eligible for the new IRAs. There are varying estimates that some $9 billion to $20 billion will flow into these accounts next year. Not all of this will be new incremental savings. But there will be enough new savings that short-term interest rates could be held down to perhaps 10 or 11 percent, he figures.
* Fewer people starting work could produce broad labor shortages this decade, encouraging companies to introduce more automation and other measures to increase productivity.
Moreover, the extra savings will provide the investment money for new plant and equipment.
McKay expects manufacturers to use more robots and other labor-saving devices. But the biggest potential for improving productivity lies in the white-collar service area where some 70 percent of the labor force now works. He expects the personal computers and office automation to spread rapidly.
This leads Mr. McKay to make an investment recommendation - Apple Computer Inc. The number of personal computers shipped to distributors, he notes, has increased from 600,000 in 1980 to about 1 million this year, and, he predicts, some 1.6 million in 1982.
''Apple is a clear winner in this area,'' he says. There already are some 350 ,000 Apple IIs in homes and offices. Software companies tend to concentrate their efforts on programs for Apple machines, where the bigger market exists. That in turn makes Apple computers more attractive to potential customers.
McKay also likes Verbatim Corporation, a maker of floppy discs for personal computers. A typical computer owner, he says, uses 10 of these discs a year. They also wear out after some time. ''These are the razor blades of the personal computer business,'' he says.
He adds: ''There are tremendous investment implications flowing out of demographics in the 1980s.''
And there are the economic implications suggesting that, despite the tough start, the 1980s may be easier to handle for Washington than the 1970s.