

My point is that you can’t build a completely teenager-proof system. Even if most parents uphold the most unimpeachable password discipline, someone’s going to put a password on a post-it note near their computer, and have their child see the piece of paper, or use their dog’s name despite their child having also met the family dog.
The original comment I was replying to was framing the issue as teenagers being allowed to watch porn versus no teenager ever seeing porn and maybe some freedom is sacrificed to do that, which doesn’t match the real-world debate. If freedoms are sacrificed just to make it a hassle for teenagers to see porn, that’s much less compelling whether or not you see it as a worthwhile goal.
As for what a teenager with access to their parents’ bank password would do, if they’re not a moron, they’ll realise that spending their parents’ money will leave lots of evidence (e.g. that they have extra stuff, their parents have less money than expected in their account, and there’s an unexpected purchase from The Lego Group on the bank statement), and so they’re guaranteed to end up in trouble for it. It’s not any different to a child taking banknotes from their parent’s wallet. On the other hand, using it to prove adulthood, if it was truly untraceable like adults would want, wouldn’t leave a paper trail.
As I said, I fundamentally disagree. Even if you can make a nearly-teenager-proof website (and so far, your example has been something that most of the people I was at school with could have beaten aged thirteen), teenagers can just go to a different website, so the system is only ever as teenager-resistant as it is difficult to find a website that doesn’t care. Most vaguely competent teenagers know how to find pirate sites with illegally-hosted TV, movies and music (even if they’re not techy, one of their friends just has to tell them a URL and they can visit it). Governments have had minimal success stopping online piracy even when aided by multi-billion-dollar copyright-holding companies, so there’s no realistic reason to think they’ll have any more success stopping porn sites with non-compliant age checks.