Your local adult video store can't sell to minors. They have to check id. If they don't, and a minor buys some goods, the store is liable.
Your local strip club can't let minors into their doors. They have to check id. If they don't, and a minor gains entry, the club is liable.
The same applies to all age-restricted products everywhere, online or offline. I can't buy liquor on doordash without showing ID for crying out loud.
But for some (probably nefarious) reason, online adult content providers want to pretend the rule doesn't apply to them. Device-level controls? Hilarious. What... are we going to add device-controls to a teenager's car to prevent them from driving to liquor stores too?
Don't get me wrong, there's some shady stuff behind AV. But I strongly disagree with the notion that an adult content provider isn't responsible for restricting their content in a way that works.