TikTok is known for having a large teenage audience, but the app is also growing in popularity among adults in the US.Brands with products that only adults would buy are beginning to test the video app as a marketing platform.Tally, a financial technology company that makes apps for automated debt management and savings, is running sponsored videos on TikTok through deals with creators and the company's self-serve ad platform.The cost to get a user to download the Tally app is 300% cheaper when running ads on TikTok versus Instagram, the company said.Click here for more BI Prime stories.As TikTok grows in popularity among all age groupsnot just 15-year-oldsit's drawing in a new set of advertisers that typically cater to adults. Tally Technologies, a financial technology startup with $92 million in venture funding, is running sponsored videos on TikTok to promote its credit card debt management app. The company, whose target demographic is individuals over the age of 18 in the 30 states where it operates, said it's seen better ad engagement on TikTok than Instagram."When we started to talk about testing out TikTok, it was a lot of people saying, 'Isn't that just for kids''" said Nick Weinberg, a senior paid acquisitions manager at Tally. "What we found from this campaign, and anecdotally, is there are a tremendous amount of adults on this platform."Tally is one of several fintech companies that are leaning into TikTok marketing. Payday-lending app Earnin, stock-trading app Robinhood, and digital currency app Sweatcoin have all run in-feed video ads on the platform."For any new social network, Gen Z and the younger generations are going to be earliest to adopt it," said Ellie Jenkins, an influencer-innovation manager at the marketing firm Mavrck. "But as TikTok continues to grow and scale and more brands join, we're definitely going to see that demographic shift and more adults begin to adopt the platform."Tally is a beta user of TikTok's self-serve ad platformTally advertises on TikTok by paying influencers to make sponsored videos using campaign-specific hashtags like #tallyup. It also uses TikTok's self-serve ad platform to place in-feed ads that include an app install button as a call-to-action.The company said it's seen TikTok outperform other social platforms when it comes to app downloads. The cost to get a user to download the Tally app is 300% cheaper on TikTok than on Instagram, Weinberg said. He declined to share how much Tally has spent on TikTok, but said it has "large paid acquisition budgets" that it divides evenly between sponsored videos from creators and its own in-feed ad placements.The company assigns age and geo-targetingfilters available in TikTok's self-serve ad toolwhen it's running in-feed ads to ensure it's reaching adults who live in the states where it operates. It casts a wider net when it comes to hiring influencers for sponsored videos, focusing on creators who have large audiences and high engagement rates.Tally looks at trending content and hashtags for its sponsored video strategyWhile a majority of TikTok users are over the age of 18, according to a leaked pitch deck from March 2019, it's impossible to run an influencer marketing campaign outside the company's self-serve ad portal without inadvertently reaching some of the 27% of TikTok users who are between the ages of 13 and 17."The only way to ensure that is perhaps working with older creators, but still I don't think that's a sure thing," Jenkins said. "The people and the creators who are driving a lot of the content tend to fall within that 13 to 24-year-old age range, and adults are currently just consumers."Like other brands that have found success on TikTok, Tally's sponsored content strategy is focused on having creators post videos around trending themes and hashtags that allow the brand to insert itself into conversations with the app's users."People taking their Instagram or Facebook strategy and implementing it on TikTok doesn't really work well," said Anish Dalal, CEO of the marketing firm Sapphire Apps, which worked with Tally on its TikTok campaign. "We follow trends first and then figure out how we connect the client or product to the trend. We look for influencers that would show up multiple times in a user's feed to make sure the ad is being seen three or four times."One trend that Tally successfully tapped into was having creators type a credit card debt-oriented message into Google Translate in order to hear a computer's voice read it back to them.As with app installs, the cost per view for sponsored videos on TikTok is much cheaper than Instagram, Dalal said."The cost per million views is 40% to 50% lower than on Instagram, and it's much easier to organically get views on TikTok," he said.For more on how brands and influencers are interacting on TikTok, check out these other Business Insider Prime posts:A milkshake brand blew up on TikTok, and its 460,000 followers have changed how it approaches marketing and its target audience: With 460,000 TikTok followers, the milkshake maker F'real has built a larger following than national brands like Chipotle, Walmart, and Burger King.A TikTok photographer explains how he gained 3 million followers in 3 months and was able to quit his job as an insurance actuary: Alexander Stemplewski quit his job to focus on social media and photography. Here are the three ways he plans to earn a living as an influencer.How TalentX plans to rule TikTok, starting with 32 influencers and a Los Angeles mansion: TalentX Entertainment is eyeing brand partnerships, merchandising, live events, and television and film development for its roster of TikTok stars.Marketers share what it's like to use TikTok's invite-only tool for finding the right influencers to hire for brand deals: Business Insider spoke with marketers who are beta testing TikTok's new matchmaking tool for influencers and brands, Creator Marketplace.Join the conversation about this storyNOW WATCH: Documentary filmmaker Ken Burns explains why country music is universal Click here to read full news..