This is to protect them from online predators.

On Wednesday, TikTok announced that it will be taking an extreme measure to protect some of their youngest and most vulnerable users. Accounts owned by those under 16 will now be private accounts by default. This means that users won’t be able to duet or stitch with videos posted by other users younger than 16. They also won’t be able to download their videos or follow them. TikTok is doing this to protect young users from online predators and groomers. TikTok did not say how many users will be affected by this, but some estimate it could be more than half of the app’s user base.