As a parent and a system administrator, I think it is a good idea to be able to mark a non-admin account with the age of the user, and have a mechanism in the browser which reports to websites if the user is adult or not. By giving to parents the technical tool to enforce age verification, this method allows them to actually enforce the responsibility that they already have.
This is much more reliable that DNS-based blocklists, which are usually outdated, and often over-reaching. And much more flexible that whitelist, which are a pain. It is also a much less invasive method, than transferring age-verification responsibility to the websites (as is the case in UK and France and some USA states), which requires the user to upload ID and selfies to unknown third parties.
Of course, it only works for legit websites, but the largest gambling and alcohol and even porn websites are perfectly legit, and have little interest in attracting minors or controversies. It would allow users to access websites which have mixed adult/non-adult content, on a per-page basis, i.e. an online store may bar minor from adding alcohol to the cart, without blocking the rest. Or a video streaming service could supply adult films, but not when accessed by a minor.
It would not work for dodgy/pirate/illegal websites, for those we would have to go back to DNS blocklists, but we already have to.
On the contrary what I would oppose, is if this scheme is enforced with a DRM-like method, locked all the way to secure boot, and age verification is enforced by vendor. It would be completely unacceptable to delegate age verification to Microsoft, or Red Hat, and have you computer locked down if you do not comply.
Tl;DR: age verification is good if controlled by the parents, bad if hard-locked and controlled by the PC vendor.