If you need an id to buy porn irl why wouldn't you need one to buy it online?
Plenty of people have been arrested for importing things legal in Japan that are illegal in the West.
Plenty of countries have laws on the books that make it a felony to even look at what's on the average Japanese store bookshelf while you're in Japan.
Why should the laws be different just because you're moving electrons instead of atoms?
You aren't going to stop teenagers from watching porn if you have the internet.
All this would do is change the websites that teenagers are using.
It strikes me as just political grandstanding since the people putting forth this idea must know it is completely pointless.
Not to mention, a website is vastly different than a physical store.