I agree, on the past I had a proposal that the OS should have a device level API to provide this information, in this case apps and (the browser as an APP) would be able to consume this information from the device. And it's up to the device manufacturer (or the OS installed) to add a age system.
This, to some extent, already exists as law in the US. In some jurisdictions, the OS must soon request the age from the user and provide it to some applications. It's not verification though, or at least it's not required to be.
Linux users could simply add a simple form with "I was born 01-01-1900" and everything would continue the same.
This started a whole war over systemd even adding a field for it (not a form). If they added a required form, it would probably start WW3.
But on the MicroSlop, Gargle and iPear worlds, they would be able to validate once and keep the information on the device.
No thanks. I'm not validating, so I'd be picking up the next Motorola phone with GrapheneOS on it instead.
Edit: to elaborate more, because the phone manufacturer is a third party and most likely sells or otherwise makes available that data to other third parties. It's also a form of deanonymization through an untrusted party.