"If the new system incorrectly identifies a user as under 18 when they are not, YouTube says the user will be given the option to verify their age with a credit card, government ID, or selfie. "

> We will use AI to interpret a variety of signals that help us to determine whether a user is over or under 18. These signals include the types of videos a user is searching for, the categories of videos they have watched, or the longevity of the account

For example, account created less than N years ago searching for skibidi -> probably teen.

So I think they can do it pretty accurately.