HERE’S A CHILLING scenario for a Scotch aficionado: You head to your local liquor store, anticipating the purchase of your favorite 12-year-old whisky, only to discover it’s nowhere to be found. You search high and low, eventually settling for a similar but slightly different (aka suspect) bottle.

This scene will soon play out in shops across the country. As a result of high worldwide demand and a finite supply of single-malt Scotch—the production of a single distillery, which can be blended only with water, not other whiskies—some drinkers’ preferred spirits are disappearing.

Lately, brands have been replacing the familiar 10-, 12- and 18-year-old range with whiskies that have no stated ages. Though aged—as all Scotch must be, by law, for at least three years—these “ageless” whiskies are younger than the whiskies they’re replacing and can be released without hitting a specified maturity.

Illustration: Brock Davis

It was really only in the 1960s that single malts started to be sold in the U.S. and in the 1990s that they began to be marketed as superior to blended whiskies (and priced accordingly), with age cited as a key indicator of quality. Perhaps this strategy was too successful.

Now, with hundreds of Scotches available, selecting a bottle without a number to latch onto can be destabilizing. While age is no guarantee of quality—many experts believe excessive time in the barrel leads to too much tannic wood flavor—in the absence of that reference point, brands must bank on consumers being educated enough to know what they like and select accordingly.

“I think you could argue from one perspective that consumers of single malts have become a bit more sophisticated than they were and don’t necessarily need to be given just numbers as a means of differentiating between different types and qualities of product,” said Dr. Nick Morgan, head of whisky outreach for Diageo. The company owns 28 Scotch distilleries, including Talisker, Oban and Lagavulin, and is currently aging 8 million casks in its warehouses.

The move away from age statements allows a brand much greater flexibility and the ability to increase supply without waiting years for stocks to age. “Age statements give you no flexibility whatsoever,” said Mr. Morgan. “They also tie your hands behind your back in terms of innovation. And innovation has always been the lifeblood of the Scotch whisky category.”

Take, for instance, Laphroaig 18-Year-Old. A few months ago the brand announced that what’s currently housed in its distributor warehouses and available in stores will be it for the foreseeable future. A number of different limited-edition whiskies will take its place. Even more unsettling, the brand says it will no longer produce the standard range of whiskies and will, with the exception of the best-selling 10-Year-Old, make different spirits available at different times depending upon supply.

In 2012 (you might want to sit down for this one), the Macallan replaced its 10-, 12- and 15-year-olds in many markets around the world with a series of ageless malts. While the brand maintains it has no plans to do the same in the U.S., it may have little choice if demand continues to rise.

“ Many of the outstanding 10-, 12- and 15-year-old whiskeys may go the way of the dodo. ”

It’s little wonder that the Scotch whisky industry as a whole can’t keep up with demand for single malt. According to the Distilled Spirits Council of the United States, sales of single malt in the U.S. rose by an astonishing 149% from 2002 to 2014. And the U.S. spends more on single malts than any other country. After decades of losing market share to vodka, wine and beer, brown spirits have been rediscovered by Americans, and producers have flooded the market with a dizzying array of them—cashing in on a growing fascination with traditionally made and artisanal products.

Back in the late 1990s, however, when today’s 18-year-old whiskies were distilled and set aside in barrels to age, forecasts for the volume of single malt that drinkers would demand in 2015 and beyond were relatively modest. Many popular single malts are no longer sold to blenders or independent bottlers. Some brands have gone so far as to buy back single-malt stocks from blenders. A number of distilleries are running around the clock; others have expanded their facilities. But the effect of these moves won’t be felt for years.

In the meantime, many outstanding 10-, 12- and 15-year-olds may go the way of the dodo. While Diageo’s Dr. Morgan assures me that whiskies without age statements are not, by definition, of any lower quality than their quantified counterparts, lovers of aged Scotch will, at the very least, need time to adjust—and whisky to drink while they do.