So there is a court ruling that the mother had sole custody. And the father tried to take the kids out of the country, possibly without the mother's knowledge. That is probably illegally moving your children abroad. The fact that they are your own flesh and blood is superseded by the mothereffing courts. Whether you agree with it or not is irrelevant. So you have to stuff the kids into a big suitcase like any upstanding CEO of an automobile conglomerate or just not leave the country.
There is a lot of meat on the Japan has catching up to do in regards to international custody battles bone. They favor the Japanese national, often unfairly, I think - although that's a topic for debate for people who are more knowledgeable. However, you can't take the law into your own hands just because you don't agree with the decisions. And CPS could talk to the Japanese authorities if they want to (and can manage). But they can do eff all. The better point of contact would be the US embassy in Tokyo. They may not be able to do anything either but if anybody can intervene it's them.

It used to be different for men and women too. That's after the women got the vote in the first place. Switzerland famously only did that in the 70s. With some women being against that. But I digress.
For men, voting age was closely linked to legal adulthood once it wasn't tied to land ownership or similar any more. Societies the world over have sort of agreed that 20-23 is about the age for that. With social progress since the 70s, it was lowered to 18. In the US because of Vietnam where 18yo were old enough to die senselessly in Nam but not old enough to elect the government that sent them there. That was unfair and addressed. In Europe, a lot of countries changed everything to 18 (drinking, voting, driving - don't do it in that order though) from the 60s onwards. And some subdivision elections (local or state level) are today from 16 in the EU.
So the question is more like why are you an adult at 18 as far as the law is concerned. That's a compromise societies have arrived at over time and often copied from one another.