Users in these countries will not be able to download apps marked 18+ unless they verify their age using "reasonable methods."
For context: in Brazil, such a label is placed on all apps that contain loot boxes.
Verification will be handled automatically on the App Store's side. Apple notes that developers may have "separate obligations to independently verify that their users are adults." For this reason, the possibility of obtaining user age information via the Declared Age Range API has been implemented.