alt Hacker News> if they have reason to believe users are underage
The law requires "clear and convincing information", not merely "reason to believe". And since the law requires developers to rely on the provide age signal as the primary indicator of the user's age, developers are not incentivized to create a system that uses sophisticated data mining to derive an estimated age. If someone posts a comment on a YouTube video saying "I'm twelve years old and what is this?", that would absolutely not require YouTube to immediately start treating that account as an under-13 account.
That would have to be litigated in court, and the easiest and cheapest way to avoid litigation is to just scan faces and IDs so you're sure your users won't upload or say anything that can bankrupt you while you sleep.