Dual citizenship

I am a US citizen and both my daughters were born in japan because i was stationed there. The have a born abroad american birth certificate. It was my belief that because they were born in japan they had dual citizenship until the age of 22 when they would have to pick one or the other (US or Japan citizenship). Would such assuption be correct?

Japanese nationality is based on whether one (or both) of the parents are Japanese. Was that the case?

I'm sure the SOFA will weigh heavily against your assumption, but it's best to ask the proper authorities.

AFAIK citizenship in Japan is only acquired by descent and naturalization. Japan does not recognize Citizenship by Place of Birth.

The only way they can be Japanese citizens is if their mother was a Japanese national at the time of their birth and they were registered in her family register. Your visa status or SOFA status is irrelevant.

yllwsmrf, what if their father was a Japanese national?

Sorry, for some reason I mistook the question asker to be the children's father. So let me rephrase my answer. At least one of the parents would have to be a Japanese national to pass on citizenship to their child, and being physically born in Japan has no bearing on the children's citizenship.

yllwsmrf, their father is half japanese and regirstered them to his family registry

Their passports also have a permission for permanent residency sticker

Half Japanese doesn't tell us much.
Is the father a Japanese National/citizen? Does he carry a Japanese passport?
If does, then the children may apply for citizenship.
But then again you mentioned
"Their passports also have a permission for permanent residency sticker"
so that somehow mean your children are not Japanese citizen then.

their father is half japanese and regirstered them to his family registry

That implies that the children are already Japanese citizens. Not sure what a "permission for permanent residency sticker" is though. What does the sticker say exactly?

Since non-Japanese can be registered in Japanese registration documents - the important question is ... Is Mother or Father a Japanese National? If yes - then, before turning 22(?) the children can elect to acquire Japanese Nationality based upon their parents Japanese nationality. Better to do it sooner, since documents may be harder to find later.