The mobile video app - which is similar to now-terminated app Vine, in which users create short videos to share with the community - has been handed the largest ever fine for a US case involving child data privacy.
The company was ordered to pay $5.7 million and implement new measures to handle users who say they are under 13, after the Federal Trade Commission (FTC) found that the Musical.ly app - which was later acquired and incorporated into TikTok - had knowingly hosted content published by underage users.
The FTC ordered TikTok to delete the data, and users in the US will now be required to verify their age when they open the app.
In a statement, the firm said: "We care deeply about the safety and privacy of our users. This is an ongoing commitment, and we are continuing to expand and evolve our protective measures in support of this."
However, TikTok also stated it would not be asking existing users in other countries, including the UK, to verify their age as the settlement only applied to the US.
TikTok's parent company, China-based ByteDance, acquired Musical.ly in 2017, and incorporated it into TikTok, discontinuing the original Musical.ly app.
The FTC said in their own statement: "For the first three years [of its existence], Musical.ly didn't ask for the user's age.
"Since July 2017, the company has asked about age and prevents people who say they're under 13 from creating accounts. But Musical.ly didn't go back and request age information for people who already had accounts."