Like any other social media
I feel like a lot of these reviews are really overreacting, and many are outdated to when the app used to be called 'music.ly' and was different.
TikTok is a social media app, where you can record video with a music/other audio backtrack and post it online for others to see. As any other social media (Instagram, Twitter, etc.), any person can post just about whatever they want, as long as it's within the app's guidelines, for other people to see.
Many TikTok videos can be fun and innocent while others, as expected, are more profane or sexual (like I said before, similarly to any other form of social media).
Most teenagers should be just fine navigating the app appropriately, especially if they can be trusted with other social media platforms.