There are two outcomes. Either the implementation is freedom and privacy respecting and very easy to bypass (effectively just a setting the OS passes on to a website) or it comes with strong technical and cryptographic guarantees which destroy privacy and freedom (identity verification, OS and hardware attestation). There is no middle ground.

The comparison to ID checks when buying cigarettes is missing the point. Human ID checks have few downsides and are relatively high cost to fool.

In the real world, you show your ID to a human and they look at the date of birth and photo. They don't copy or photograph it, they surely won't read let alone remember anything else from your ID, it would be very obvious, costly and dangerous for a criminal to install a hidden camera and secretly record everyone and their IDs. We also don't attach the ID physically to your body and assign an individual police offier to follow you around 24/7 so you don't try to tamper with it somehow.

On the Internet, a securely (safe from bypasses) implemented age verification system makes sure your device is owned and used only by you, that you can't lend it to somebody, that you can't modify or inspect it... It also enables some level of reidentification for catching and prosecuting you if enable access to a minor despite this.

These are two wildly different situations.