Like others have been saying, that was more of a technicality than a reality, as the actual age for every prefecture was already 16-18, with ages lower than 18 generally only being permissible in a legal marriage approved of by the parents (If I'm remembering right).
It's kind of like if in the US, the Federal age of consent was 13, but all 50 states and every territory had already raised it to 16 to 18, meaning the real age of consent was basically 18, it's just that congress was too lazy or distracting to update the federal law to what all the states and territories had already passed.