The company behind brain-training game Lumosity has agreed to pay $2m (£1.4m) to settle false marketing claims.
Lumos Labs had said its games helped users perform better at work and even alleviated the symptoms of Alzheimer's disease.
But the US Federal Trade Commission alleged it did not have scientific evidence to back up the claims.
The company must now contact all of its customers to offer them the chance to cancel their subscriptions.
Launched in 2007, Lumosity consists of 40 online games, purportedly designed to train specific areas of the brain.
In advertising, it claimed using the games for 10 to 15 minutes three or four times a week could help users achieve their "full potential in every aspect of life".
It also said the games could alleviate the symptoms of dementia, strokes and brain injuries.
But the FTC's Bureau of Consumer Protection director, Jessica Rich, said: "Lumosity preyed on consumers' fears about age-related cognitive decline."
The games had been "widely promoted" though TV and radio ads on networks including CNN, the History Channel and Fox News
But Lumos "simply did not have the science to back up its ads", Ms Rich added.
The FTC said Lumos had also failed to disclose that some consumer testimonials on its website had been solicited through contests that promised prizes, including a free iPad.
But Lumos Labs told the BBC: "Neither the action nor the settlement pertains to the rigor of our research or the quality of the products - it is a reflection of marketing language that has been discontinued.
"We remain committed to moving the science of cognitive training forward and contributing meaningfully to the field's community and body of research."
The FTC had wanted to fine Lumos Labs $50m, but said it was accepting the smaller sum of $2m because of the company's "financial condition".
The company must also offer customers "an easy way to cancel their subscriptions", which range from $15 to $300.
In January last year the company said it had 70 million members worldwide.