To marry someone in Japan males have to be 18 years or older and females have to be 16 years or older. Those under 20 have to have parental consent though. A woman has to wait at least 6 months after divorce to remarry supposedly so that if she has a child, there won't be confusion as to whom the father is. Prior to 1995, the marriage age was 16 for males and 13 for females. Article 177 of the Penal Code puts the age of consent for sexual activity at thirteen years. Though many areas have it set as a higher age, for example currently the age of consent is 17 in Tokyo (Youth Protection Law).
If you wish to marry in Japan, you will do so according to Japanese law. Marriage in Japan consists of a civil marriage registration by the couple at a Japanese municipal government office. Only civil registration constitutes a legal marriage in Japan. Ceremonies performed by religious or fraternal bodies in Japan are not legal marriages.