In a viral audio clip on TikTok, a white-haired man in a baseball cap and polo shirt declares, “The millennials and Generation Z have the Peter Pan syndrome, they don’t ever want to grow up.”

Thousands of teens have responded through remixed reaction videos and art projects with a simple phrase: “ok boomer.”

“Ok boomer” has become Generation Z’s endlessly repeated retort to the problem of older people who just don’t get it, a rallying cry for millions of fed up kids. Teenagers use it to reply to cringey YouTube videos, Donald Trump tweets, and basically any person over 30 who says something condescending about young people — and the issues that matter to them.

Teenagers have scrawled the message in their notebooks and carved it into at least one pumpkin. For senior picture day at one Virginia high school, a group of nine students used duct tape to plaster “ok boomer” across their chests.

The meme-to-merch cycle is nothing new, but unlike most novelty products, “ok boomer” merch is selling. Shannon O’Connor, 19, designed a T-shirt and hoodie with the phrase “ok boomer” written in the “thank you” style of a plastic shopping bag. She uploaded it to Bonfire, a site for selling custom apparel, with the tagline “Ok boomer have a terrible day.” After promoting the shirt on TikTok, she received more than $10,000 dollars in orders.

“The older generations grew up with a certain mind-set, and we have a different perspective,” Ms. O’Connor said. “A lot of them don’t believe in climate change or don’t believe people can get jobs with dyed hair, and a lot of them are stubborn in that view. Teenagers just respond, ‘Ok, boomer.’ It’s like, we’ll prove you wrong, we’re still going to be successful because the world is changing.”

Ms. O’Connor is far from the only one cashing in. Hundreds of “ok boomer” products are for sale through on-demand shopping sites like Redbubble and Spreadshirt, where many young people are selling “ok boomer” phone cases, bedsheets, stickers, pins and more.

Nina Kasman, an 18-year-old college student selling “ok boomer” stickers, socks, shirts, leggings, posters, water bottles, notebooks and greeting cards, said that while older generations have always looked down on younger kids or talked about things “back in their day,” she and other teens believe older people are actively hurting young people. “Everybody in Gen Z is affected by the choices of the boomers, that they made and are still making,” she said. “Those choices are hurting us and our future. Everyone in my generation can relate to that experience and we’re all really frustrated by it.”

“Gen Z is going to be the first generation to have a lower quality of life than the generation before them,” said Joshua Citarella, 32, a researcher who studies online communities. Teenagers today find themselves, he said, with “three major crises all coming to a head at the Gen Z moment.”

“Essentials are more expensive than ever before, we pay 50 percent of our income to rent, no one has health insurance,” said Mr. Citarella. “Previous generations have left Generation Z with the short end of the stick. You see this on both the left, right, up down and sideways.” Mr. Citarella added: “The merch is proof of how much the sentiment resonates with people.”

Rising inequality, unaffordable college tuition, political polarization exacerbated by the internet, and the climate crisis all fuel anti-boomer sentiment.

And so Ms. Kasman and other teenagers selling merch say that monetizing the boomer backlash is their own little form of protest against a system they feel is rigged. “The reason we make the ‘ok boomer’ merch is because there’s not a lot that I can personally do to reduce the price of college, for example, which was much cheaper for older generations who then made it more expensive,” Ms. Kasman said. “There’s not much I can personally do to restore the environment, which was harmed due to corporate greed of older generations. There’s not much I can personally do to undo political corruption, or fix Congress so it’s not mostly old white men boomers who don’t represent the majority of generations.”

Ms. Kasman said she plans to use proceeds to pay for college. So do others.

“I’ll definitely use the money for my student loans, paying my rent. Stuff that will help me survive,” said Everett Solares, 19, who is selling a slew of rainbow ‘ok boomer’ products. “I hadn’t seen any gay stuff for ‘ok boomer,’ so I just chose every product that I could find in case anyone wanted it,” she said.

Gavin Deschutter, 17, reimagine famous logos for companies like FedEx, Budweiser, Google, and KFC with the catch phrase, and has been selling t shirts and phone cases emblazoned with the message. He hasn’t made very much — “I sold a hoodie yesterday for $36,” he said — but his designs have been shared across meme pages on Instagram.

Every movement needs an anthem, and the undisputed boomer backlash hymn is a song written and produced by Jonathan Williams, a 20-year-old college student. Titled, inevitably, “ok boomer,” the song opens with: “It’s funny you think I respect your opinion, when your hairline looks that disrespectful.”

The chorus consists of Mr. Williams screaming “ok boomer” repeatedly into the mic. Peter Kuli, a 19-year-old college student, created a remix of the song, which has seen 4,000 TikToks made from the track. The two planned to split the revenue earned through streams of the song on Spotify.

“The song is aggressive and ridiculous, but I think it says a lot about Gen Z culture,” said Mr. Kuli. “I think because of the internet, people are finally feeling like they have a voice and an outlet to critique the generations who got us into this position.”

“Millennials and Gen Xers are on our side, but I think Gen Z is finally putting their feet in the ground and saying enough is enough,” he said.

Teens say “ok boomer” is the perfect response because it’s blasé but cutting. It’s the digital equivalent of an eye roll. And because boomers so frequently refer to younger generations as “snowflakes,” a few teenagers said, it’s particularly hilarious to watch them freak out about the phrase.

“If they do take it personally, it just further proves that they take everything we do as offensive. It’s just funnier,” said Saptarshi Biswas, 17.

“Instead of taking offense to them, you’re just like, ha-ha,” said Julitza Mitchell, 18.

In the end, boomer is just a state of mind. Mr. Williams said anyone can be a boomer — with the right attitude. “You don’t like change, you don’t understand new things especially related to technology, you don’t understand equality,” he said. “Being a boomer is just having that attitude, it can apply to whoever is bitter toward change.”

“We’re not taking a jab at boomers as a whole — we’re not going for their lives,” said Christopher Mezher, 18. “If it’s a jab at anyone it’s outdated political figures who try to run our lives.”

“You can keep talking,” Ms. Kasman said, as if to a boomer, “but we’re going to change the future.”