i found http://legal.3yen.com/2005-07-27/is-the-age-of-consent-in-japan-really-13/
which says that it's 13 but complicated. However, please remember that you live in the lovely US of A and can get charged for writing child pornography if any of your characters are underage. Just do a time-bending thing and make your underage kiddo an adult. It will make your life easier.

And don't forget to share... :-)