I know systems thinking, and am in favor of a version of these types of legislation. Give me your best argument from systems thinking, and I'll give you a thoughtful reply.
The only reasonable solution shape I've seen is the one that trusts the parents to set an operating system setting that says whether or not the user is allowed to access adult content. And so it doesn't actually verify age, it just verifies parental intent.
The argument usually is that it is a slippery slope. Something that is introduced in the name of virtue ends up being co-opted into a system of control as those in power and peoples attitudes change with subsequent layers of normalization.
A significant part of the cultural value of the internet comes from free anonymous expression. As a key example, look at 4chan - anonymity taken to it's extreme has resulted in on one hand yes a lot of disgusting stuff, but also a cultural hotbed.
Age verification is de facto identity verification. Eff says it well:
> But no matter the method, every system demands users hand over sensitive and immutable personal information that links their offline identity to their online activity.
https://www.eff.org/issues/age-verification
Tying every action taken online to the user's real identity will have a deep and catastrophic chilling effect, destroying those very places that are creating our culture.