It would be a rather strange law, but it certainly would explain some things, like how even in isekai series the MC always refuses to drink alcoholic drinks, even though it's an entirely different universe. Such a law would also mean that if in a manga a Japanese young adult of 18 or 19 summers travels to Europe, the manga couldn't have the character drink a cold one. More importantly, wouldn't it mean the manga couldn't depict any European young adult drinking either, even though that would definitely break the realism, no matter how realistic the manga might otherwise try to be.