The revised national referendum law took effect Friday, setting the stage for the minimum voting age to drop to 18 from 20 in four years.
The law, enacted by the Diet a week ago, is seen as a crucial part of Prime Minister Shinzo Abe's push to amend the pacifist Constitution to allow Japan to play a greater security role.
Changes to the Constitution can be initiated with the support of at least two-thirds of the lawmakers in both houses of the Diet and must be endorsed by a majority of voters in a referendum.
The minimum voting age for national referendums will be lowered to 18 in four years' time under the legislation, which had the support of the ruling Liberal Democratic Party and its coalition partner, New Komeito, the main opposition Democratic Party of Japan, and five other parties.
The issue of formally amending the Constitution, which has never been revised since its promulgation in 1946, continues to be a source of conflict among the parties and is currently considered politically unviable.
Abe wants to enable the Self-Defense Forces to come to the defense of allies under armed attack by exercising the U.N. right to collective self-defense, which is banned under the government's current interpretation of the supreme law.