The thing about doing age verification at the OS level is the user could just install a crack that rewrites the necessary code. It'll take some heavy DRM type stuff to block that. Possibly hardware support, like a specialised TPM.
No way can that be standardised and then rolled out quickly. If they rush it then it'll be some proprietary power grab.
The alternative is each website and app does it separately which will be spotty and provide endless security breaches.
It'll be a shitshow either way.