Comment by sajithdilshan

The problem is that it would be a client side identity verification mechanism and won't work for websites.

As an example this would means that if this exists in iOS, everytime a safari loads a webpage it has to send this additional verification identifier as a header or a query param to the web server that serves the request and based on that the web server deny the request if the identifier is present (that means a minor is trying to access the web page).

But this client side identifier can easily be bypassed via a proxy that strips the identifier from the request because not matter it's configured in the OS of the device, ultimately request would just goes out as HTTP and that can be manipulated. That's why it has to be a server side verification of the age/identification.