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 

I'm pretty sure I remember reading that you can have dual citizenship for a while, but then you have to choose one. I think it's at 20.

0xbitcoiner

Yes, you need to renounce your old citizenship.

But l believe some prople can be dual depending on the conditions.

charts_and_numbers

Ranked: Top Countries by Population of Chinese Immigrants

Ah, you mean changing citizenship. 
Yes, you need to renounce your old citizenship. 
But l believe some prople can be dual depending on the conditions. 