I guess the real question now is whether or not they set up any system of locating, arresting and punishing the new "criminals" (aka giving the law teeth) or if they just made it illegal to look good.
The beer vending machines where I live all have ID scanners on them, so underage drinking is that much harder, but one thing that's still all over the place is prostitution. Yeah, it's technically illegal, but the brothels are packed into a single large neighborhood here with giant neon signs all over the place, and nobody does a damn thing about it. The only time the cops come around is when someone sets up an illegal casino in one of the basements.
Quick tip for future Japan dwellers: A guy from the NHK TV broadcasting company will come around to your house and insist that it's illegal for you to have a TV in your home and not be paying NHK every month. This is true, but if you refuse to sign the contract, they have zero legal recourse against you. If he starts arguing with you, start talking to him about god, and he'll go away. If you DO sign the contract, on the other hand, then they can technically sue you and get away with it.