I was wondering what was the legal age to be considered as an adult in Japan. In most Western countries it's 18.

The coming-of-age ceremony 成人式 (seijinshiki) is at 20. It's also the legal age for drinking and smoking.

However, I have been told that the legal age to have a driving licence is 18, like in Europe (and unlike America). It seems that you can also have a passport and travel alone at 18, not 20.

What's the legal age to sign documents, contracts, etc. without a tutor ?

The term "coming-of-age" is confusing. For me it means be able to sign alone and take full responsibility of one's acts, be able to drive, travel abroad alone, or be independant from my parents. It doesn't mean to drink or smoke as there is no legal age for this in some European countries (eg Belgium) and in others its much earlier than 18 (can't imagine a party without a minimum of alcohol even when you are 14 or 16).

So, I don't understand why Japanese people insist that they become adult (seijin) at 20 and not 18 if they are legally independant (to sign, contract, drive, travel on their own, etc.).

Can anybody give me more information about this ?