Recent posters here are clear that porn sites are setting every available signal that they are serving adult-only content.
According to them, you are targeting the wrong audience.
Facebook/Instagram studying how to get young users addicted should be of greater concern. I have my doubts about the effectiveness of age-based blocking there, though.
> give parents the ABILITY to advertise the users age to browsers, apps and everything in between.
Accounts and Applications to services that provide countent are set to a country-specific age rating restrictions (PG, 12+, 18+, whatever). That's it.
None of the things you mentioned have any point to concern themself with the age or age-bracket of the user in front of the device. This can and will be abused. This is very obvious. Think about it.
Responsibility should be on the website to not provide the content if the header is sent with an inappropriate age, and for the parent to set it up on the device, or to not provide a child a device without child-safe restrictions.
It seems very obviously simple to me, and I don't see why any of these other systems have gained steam everywhere all of a sudden (apart from a desire to enhance tracking).