Japan does not consider someone an adult capable of making that decision until they are 20. Further, they won't force the decision (nor will the US) even after

Japan does not consider someone an adult capable of making that decision until they are 20. Further, they won't force the decision (nor will the US) even after that point. This leads to interesting issues. One such individual was in training with me but had to sit and wait 6 months until he turned 20 to renounce his Japanese citizenship so that he could obtain a clearance. This was 20 years ago, so things may have changed.