For the present, Japan's legal age for attaining the majority is 20; those who have not reached 20 are in principle regarded as minors. Banks have to check the mind of your parent as a statutory agent for you, because you are considered to be a minor.
As to a bank account under the name of a minor [you], the statutory agent [your parent]
either makes deals regarding the account,
or agrees, with responsibility, to the minor's [your] making deals regarding the account.
Shinsei Bank requires that one statutory agent [your parent] has already had an account in the bank by the time when a minor [you] opens an account, presumably for the purpose of reducing the risk of camouflage deals. (A person who apparently has nothing to do with a bank can more easily try camouflage deals than a person who has an account in the bank.)
Deals regarding a bank account accompanies juridical acts which can be rescinded [invalidated] later if performed by a minor against his/her statutory agent's mind.
While you are a minor, you can not open or handle a bank account without permission or consent of a major.
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