哪里有写六岁For children under the age of 18 without a valid vaccination certificate, if they are accompanied by a guardian with a valid vaccination certificate and who will supervise the children, they will be treated as holders of valid vaccination certificates, and submission of the negative certificates is exempted the same as the guardian as an exception.
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In case the accompanying guardian does not hold a valid vaccination certificate and enters Japan with a negative certificate, preschool-aged children (generally under the age of 6) are not required a negative certificate.