https://techcrunch.com/2018/07/19/facebok-under-13/

Facebook and Instagram will more proactively lock the accounts of users its moderators encounter and suspect are below the age of 13. Its former policy was to only investigate accounts if they were reported specifically for being potentially underage. But Facebook confirmed to TechCrunch that an “operational” change to its policy for reviewers made this week will see them lock the accounts of any underage user they come across, even if they were reported for something else, such as objectionable content, or are otherwise discovered by reviewers. Facebook will require the users to provide proof that they’re over 13, such a government-issued photo ID, to regain access. The problem stems from Facebook not requiring any proof of age upon signup.

Facebook Messenger Kids is purposefully aimed at users under age 13