Fears about cord-cutting sparked a massive media industry-wide stock sell-off earlier this summer, but a new report highlights a burgeoning group that could pose an even bigger threat: “cord-nevers.”

The study, from Forrester Research, found that cord-nevers – people who have never subscribed to a traditional pay-TV service – make up 18% of the U.S. population. While most cord-nevers are age 32 and above (people who have grown up in the familiar pay-TV paradigm but eschewed it anyway), about 7% of the U.S. population are cord-nevers between the ages 18 and 31.

That figure is larger than the 6% of the population who are cord-cutters, or people who have cancelled their cable or satellite subscription, Forrester said.

Forrester dubs that younger subset of people who have never had a pay-TV subscription “digital cord-nevers.” In general, nearly half of digital cord-nevers use Netflix and YouTube, Forrester said.

“While older cord-nevers understood that they would have to live without access to some of the best programming by forgoing pay-TV service, younger digital cord-nevers have grown up believing that they can have all of the TV they want without paying a traditional TV distributor for it,” Forrester analyst James McQuivey writes in the report.

Forrester estimates that by 2025, half of all TV viewers under age 32 will not pay for TV in the current model. Still, Mr. McQuivey writes that the “rumors of the death of pay TV are greatly exaggerated.” About 76% of U.S. adults are “cord-havers.”

Forrester predicts that, as a result of these trends, programmers will eventually have to crack down on things like password sharing, which encourages greater use of streaming services in the short term but also gives away content for free and prevents services from correctly identifying who is watching.

The study, which surveyed 32,000 U.S. adults, concludes that content owners have an upper hand against pay-TV operators for one main reason: they make the shows and ultimately control their distribution.

“There are 12 ways to watch ‘Game of Thrones,’ but only one company can sell the rights to distribute ‘Game of Thrones’ in those 12 ways,” Mr. McQuivey writes.

Write to Steven Perlberg at steven.perlberg@wsj.com