> If I'm a parent, how do I prevent my kid from telling some website that they are an adult?
You don't need to because all content filters and permissions can be implemented on the client-side at the OS and app level. Rather than every service taking your child's age and deciding what to filter, you decide what to filter based on your child's level of maturity. You'll have total control over what runs on the hardware. Think: user accounts and permissions, local firewalls and traffic inspection, executable permissions, app installation locks, stripped down browsers and apps, small local neural networks, bootloader locking with custom password, etc. The market will follow the gradient of user value and spawn a bunch of OSes with different feature sets, probably better than what I can come up with, and you pick the OS with features that suit your needs.