Or realize that there is no way to know who is using a device without looking at the device holders for all the time they use the device. It's like ordering alcoholics or cigarettes: the seller looks at you see the white hairs (OK) or see a 16yo face (documents please.) A device registered to an adult can be used by a child. I know, it's a failure of parental control, but how many teenagers managed to get adult magazines in the last 100 years? They will get adult devices too. So there is no way to create a 100% effective system. Kids and teenagers will find workarounds, everybody else will live a worse life.