Board Thread:Questions and Answers/@comment-4098937-20160110112939/@comment-35248429-20180731035957

Japan's national age of consent is 13, but individual prefectures have their own ages of consent that are generally much higher. Tokyo's is 17 or instance.

https://www.ageofconsent.net/world/japan

As stated, it wasn't in the beta or the manual for the game. I'm guessing it was added by kayaba as another way to make SAO an actual reality