The age for marriage is considerably different from country to country and in Japan the age for marriage has been 16 for female and 18 for male.
The law which stipulates the age for marriage was changed recently and now the age is 18 years old for both male and female. The low age of content is Japan in known internationally and it has been controversial over the last few decades. In Japan the age of consent is 13 and this is quite low compared with the other countries of the world. Many people elsewhere on the globe may misunderstand that this is the actual age of consent and want to know why it is so low. However, having sexual relationships with minors are banned by various laws. Learn more: https://www.jluggage.com/blog/law/what-is-the-age-of-consent-in-japan/

Sexual intercourse with a minor, even if the other party consented, may violate Child Welfare Law, Act on Punishment of Activities Relating to Child Prostitution and Child Pornography, and Protection of Children, etc., and youth development ordinances established by each prefecture. There are various laws.
A minor under the Civil Code used to be defined as a person under 20 years of age (Civil Code Article 4), but a child or adolescent whose sexual activity was problematic was defined as a person under 18 years of age. Now the definition of a minor is a youth under 20 years old because of the amendment of the law.

The laws or ordinances that are in conflict vary depending on the age of the minor, whether or not payment is made, and the manner in which the sexual act is performed.
In addition, there are various cases in which such a case triggers for investigation, such as when a person with parental authority over the other minor notices the child's behavior and consults the police, or when the other minor becomes involved with an investigative agency in another case. If such a case is brought to the attention of the police, whether or not a settlement has been reached with the other woman and her guardian may greatly affect the outcome of the criminal punishment.