After the boom comes the bust -- not in stocks but in population. The leading edge of the baby bust, that smaller generation born since 1965, is now 23 years old and starting its long march through maturity. It may have a far more profound effect on the economy than the recent drop in the Dow Jones industrial average.

All eyes remain fixed on the baby boomers because of their rising potential to save and spend in middle age. But the huge expansion they fueled is over.

Throughout the 1970s, greater numbers of 23-year-olds surged toward adulthood every year, peaking in 1983 at 4.4 million. They needed more, much more, of everything.

But now you're seeing smaller and smaller numbers of 23-year-olds -- needing less, in the aggregate, of what it is that young adults buy. There are 5.9 million fewer teen-agers today than there were 10 years ago. That will translate into 5.9 million fewer car buyers, 5.9 million fewer first-home buyers, 5.9 million fewer people to read newspapers, watch network TV, use gasoline, drink Pepsi.

"Our present economy is built for a more rapid increase in demand than we're going to see in the future," says economist Jay Levy, publisher of Industry Forecast. Long term, businesses adjust to population drops. (Baby-food companies, for example, have diversified.) But over the short run, specific industries can get hit.

It stirs up an argument to ask whether the baby bust will be good or bad for the economy. On the face of it, fewer young workers will mean less consumer demand. On the other hand, their smaller numbers should bid up wage levels and raise disposable incomes.

Specific predictions based on demographics are slippery at best, because they can be overwhelmed by changes in social and economic behavior. But when I asked analysts, "Where's the money in the baby bust?" they pointed to the following trends: Look at capital-equipment stocks, especially in computers and communications. As labor grows more expensive, companies will buy labor-saving machinery.

"A whole new crop of capital-intensive growth companies should emerge," said economist Walter Cadette, a vice president at Morgan Guaranty Trust. Meanwhile, labor-intensive, low-wage industries, such as textiles and retailing, will be hurt, as the shrinking pool of workers heads for better jobs.

Don't buy fast-food partnerships or restaurants. Franchisees have been pressing retirees into the gap as the supply of teen-agers shrinks, but labor costs will inevitably rise. The big question is on the demand side: Will an older population support as many Burger Kings and Pizza Huts?

It's too soon to worry about the auto industry, said auto analyst Mary Ann Keller of Furman Selz. The heaviest car-buying years fall between the ages of 35 and 45, which is just where the hump of the boomers is heading.

Older people also have more income than they used to and are buying more cars. Any slowdown in demand arising from the baby bust may be a good 10 years away.

Don't invest in real-estate partnerships that build small town houses, garden apartments or any other kind of starter housing. There is going to be a glut on the market, which will hold down the price.

That's bad for baby boomers who will want to sell their first homes to trade up to something better. But it's good for busters, who should find it easier to buy.

Expect serious money to flow toward investments in child care. If too many mothers withdraw from the labor force because they can't find decent care for their children, the shortage of workers will grow even worse. To a corporation, subsidized day care will start looking cheap compared with the alternatives.

The leading edge of the busters may not be affected by these trends because they're still blocked by a wall of boomers.

But as the decade wears on and young adults thin out, their prospects should brighten. This news is especially good for skilled women and minorities. So many hands are going to be needed, no one will care what color they are.