Public transportation in Japan is great. In urban areas it's also quite easy, accessible, and affordable to rent a capsule hotel "room" or net cafe seat. This is true no matter what your age or credit history, they won't ask for your drivers license, you can pay cash. It's honestly strange that I have to state some of these things, but I live in the U.S., and this country is all kinds of screwed up.
Also, the enforcement and norms for drunk driving (setting aside age entirely) are totally different. From my experience it's not common to have _a_ drink and drive, and the limit is "detectable".
For a driver in the U.S., the legal and cultural expectations are that you assess your own inebriation and decide when it's okay and when you need to get a cab. That standard doesn't really exist in Japan.
While I might not agree, I can understand people's hesitation about putting the U.S. standard on people aged 18 and 19. But Japan's standard? Pure nanny state.