There is a simple and better way to do this, which is device-wide age status attestation. That is, the whole device or user account has a 'minor' flag set, and passes it on to software, and so on.
Governments are not pushing for this because this is not about protecting children, it is about removing privacy and increasing control.
User agent flags? Those are some of the easiest to spoof. Wouldn't last 10 minutes.
That doesn't really matter if something like more than 95% of time it's followed. Compared to now where there is nothing.
No, there is hardware based attestation, and it could be set by an administrator / parent for the user account.