Unfortunately there's no real way to ban is because as others have already said, kids can get round it by using a VPN or by using their phones and a ' warm ' connection. Kids are more techno savvy than we give them credit for.
The other problem is with the age verification, shy of someone having to verify their age with photographic proof, such as a passport or driving licence, then it's just a fake birth year, which is no proof at all, and again kids are savvy.
The problem with porn itself today is twofold. One is the content, which varies from mild straight vanilla sex to degrading violent stuff, with 'teens', paedophillia, forced sex, anal and god knows what, and it's everywhere! Even mainstream TV doesn't escape it.
The second problem is the accessibility to this stuff. Yes, once upon not so long ago it would have been a mucky mag from the top shelf or a blue movie from the bloke in the pub, which in comparison to today's stuff was pretty tame.
Now we have the other problem of young girls being coerced into sexual acts because, due to porn they think what they're seeing is normal as they have nothing to compare it to, as it's all they've seen.
Not too many years ago, anal sex was literally unheard of among a straight couple, mainly because it was illegal and because no one owned up to doing it, even if they were. Now it's in literally every porn clip and so has become 'normal'.
The same applies to boys, but with the addition of them pressuring the girls into certain acts, because again, they have nothing to compare it to. Therefore he will pressure a girl into doing anal because it's 'normal' and everyone does it, in his mind.
It's damaging and dangerous, very much so to a young impressionable mind.
That said, I have no idea what the answer is to stem the tide and turn things around, except keep talking to your kids.