In the same way parents can be blamed for not keeping their children safe around guns/alcohol/drugs, they should also be blamed for not keeping the children out of digital dangers, and keep mandatory age verifications out of here.
The premise that parenting is wholly on the parents and society at large doesn't need to play any role in raising kids is a manifestation of the kind of libertarianism that appeals to techies on the spectrum who want to find the simplest possible ruleset for everything, but it just doesn't work that way in reality.
The government can and does already track whatever they want about you. Businesses already track you unless you are extremely thorough about erasing your footprint. Adding a zero-knowledge proof through a trusted system that you are 18+ doesn't seem like the mountain people are claiming. You already have to provide ID and credit card to get ISP access, the byte patterns are traced back to your household. They already have a unique fingerprint on your browser and computer. The real harm is just the obvious encroachment that we can all see and have known about since early 2000s. They don't need a "backdoor", it feels like alarmism over a possible problem, when there is a very real harm to children and teens (suicide rates, depression, bullying, mental health, etc).
to go back to smoking / alcohol / guns, one could argue it is an infringement, but ultimately it does seem to have been the right choice for society at large, and the increased "invasion of privacy" has been pretty minor. If anything, the opt-in stuff like credit cards, cell phones, GPS, car apps, streaming services have all been far larger invasions of privacy that people willingly embrace.
Also, the fact that gov and companies are already tracking people doesn't mean we should consent to more ways of tracking.