This puts Musical.ly in a strange position. Websites and online service operators that target users under 13 must meet federal requirements regarding the collection and sharing of personal information, which is defined broadly to include names, photos or videos, or persistent identifiers, such as user names. The restrictions are part of the Children’s Online Privacy Protection Rule, often called Coppa, enacted by the Federal Trade Commission.  Like most social networks that operate in America, Musical.ly’s terms of service prohibit users under the age of 13. But the app doesn’t collect age information about its users, a convention often used by networks that, at least anecdotally, are widely used by children. (Those networks that do ask for an age, like Facebook, tend to take the user’s word for it.)  “The approach taken by most operators is to avoid triggering Coppa — to claim ignorance,” said Denise G. Tayloe, chief executive of Privo, a company that assists online services with Coppa compliance. Enforcement is both difficult and relatively rare; the rule neither requires social platforms to screen for age, nor does it hold companies accountable for users reporting false ages.  According to a spokesman, the F.T.C., as a law enforcement agency, cannot comment publicly about individual companies. In an email, the agency said: “We support the development of robust, easy-to-use mechanisms companies that collect kids’ personal information can use to seek the parents’ consent.”  Services that are more openly marketed toward children often stringently adhere to Coppa’s privacy rules. Vine Kids, for example, is a limited and largely passive service with no user names or video posting capabilities; similarly, YouTube Kids is essentially an app full of streaming children’s programming, walled off from the rest of YouTube’s ecosystem. In contrast, Musical.ly, like Snapchat or Instagram, is a full-functioning social network, popular with young people but not openly marketed to them.  Such discussions about privacy can feel strained against the backdrop of technological change. The first version of Coppa became law in 1998, almost a decade before the iPhone was introduced. Last year, the research firm Influence Central said that, on average, parents who give their children smartphones do so at age 12. And once they have a phone, they get apps.  In a study of the law published in 2011 by the academic journal First Monday, researchers suggested that Coppa created intractable issues. To remain compliant, tech companies either cut off young users or claimed ignorance of their presence, while parents, for whom the law is meant to provide guidance and comfort, often ended up helping their children circumvent sign-up rules. Increasing the current style of enforcement, the report concluded, would only encourage firms to “focus on denying access rather than providing privacy protection or cooperating with parents.”  In short, children are using their smartphones much like the rest of us, whether or not they are comprehensively addressed by regulations or by broader cultural conventions.  Alex Hofmann, president of Musical.ly, said the company tries to be mindful of its popularity with younger users. “One of the differences to other apps,” he said, “is that we don’t only talk to the musers” — the company’s term for users — “we talk to the parents.”  He keeps close counsel with a network of a few dozen top users, and some of their families, and frequently asks for feedback from both regarding everything from user safety to new features. (The company’s support page contains an entire section directed toward parents — one that notes the app is “intended for 13+ only.”)  Ultimately, Mr. Hofmann said, he expects the app to diversify its audience. “We really see ourselves as a real social network, and as a network for different age groups,” he said.  For now, the company will have to navigate a peculiar if widely envied situation — capitalizing on its apparent popularity with an audience that it cannot fully acknowledge, watched over by wary but increasingly complicit parents.  “A year ago, there was basically nobody who was 40 years old on Snapchat,” Mr. Vaynerchuk said. “If Musical.ly can hold on, they will age up.”  Asked whether an app attracting such a young audience might be a liability, he cast the matter of children using social apps less as an argument than as an inevitability. Children are using these apps, he said. And besides, in the beginning, it was the children who chose Musical.ly, not the other way around.  What remains to be seen, he said, is whether any particular service is too much, too soon — something that is largely out of the company’s hands.  “There’s no campaign, there’s no money to be thrown at it, it will just become something you get used to,” Mr. Vaynerchuk said. “There’s nothing to address. This is about social norms.”  A version of this article appears in print on September 17, 2016, on page B1 of the New York edition with the headline: Who’s Too Young for an App? Musical.ly Tests the Limits. Order Reprints| Today's Paper|Subscribe
The video-sharing app Musical.ly seems to be popular with children well under the age of 13, which leads to questions over how to monitor young people on the web.