My children have dual citizenship as of now. However, my daughter must choose her nationality before 22 years of age: https://www.moj.go.jp/ENGLISH/information/tcon-01.html

My boy has to decide even earlier, when he is 16.5-17 years old. This is because we have mandatory military service starting from 18 years of age onwards. So he must decide whether he wants to serve Singapore as a soldier haha