In the context of age limits, that is wrong. The German eID has a zero knowledge method of proving that your age is above a certain number without revealing anything else. That method has been around for like 15 years and these days, thanks to smartphones with NFC readers, is quite user-friendly.
In practice it's basically not used anywhere except for cigarette vending machines because it's much simpler to hire some dubious third party "wave your ID in front of your camera" service
Edit: mandatory age verification is still an atrocious idea for a number of other reasons, just to be clear
I won't use the eID because I don't believe in its promises. I don't need a third party, which would be completely dependent on government, to put a signature on my net access.
I would even prefer the dubious service because of the relationship dynamics I mentioned. Best case is that age limits for the net should be enforced on device by parents. Problem solved, no unnecessary infrastructure needed.
Theoretically you could have anyone sign and attest to your age at any time. So maybe the government gives you an attestation of 0 at birth, with timestamp (allowing age to be calculated at any time), as part of the normal new-human bureaucracy. And/or maybe you can separately hire an accredited (co-signed?) lab to perform carbon dating on you later on :)
I totally would prefer the biopsy to a government Id. So carbon dating here I come.