Japan has been a functioning democracy for longer than any state outside of Western Europe or the Western European settler colonies. It had a functioning but fragile democracy from shortly after the beginning of the Meiji period until it ended after the Great Depression. That places it on company with Germany and Italy.

Could you expand on the aspects you think are remnantis of its past? The Imperial Family and Household are not mentioned in the constitution and there’s no aristocracy. I’m sure it’s dominated by Tokyo and the national government but it seems unlikely to be more centralised than England, or the U.K., tout court.

There are many democracies younger than Japan.

Yes, basically all the ones coming from the decolonization era. Most of them are pretty unstable, unfortunately.