Can a 18-year-old Japanese citizen travel alone from Japan to South Korea? Japanese citizens are not currently considered full adults until age 20.
Yes. JAL for example allows unaccompanied travel from age 5 upwards.
What age does my child have to be to qualify as an unaccompanied minor?
Unaccompanied minor service is mandatory for a child age 5-11 traveling without an adult. Children ages 12-17 years are not required to travel as unaccompanied minors, but the service is optional for this age group as well.
Children under 5 years of age are not allowed to travel unless accompanied by an adult
Of course you can. Age for flying alone are typically set by the airline, not the country (as long as all visa requirements are met, that is).
Typically they require some sort of extra service for kids 14 and under, but 15 and up can travel alone. Here is JALs policy https://www.jal.co.jp/jba/aa/en_US/specialAssistance/childrenTraveling.html
My kids have certainly traveled internationally on their own when starting at 15 or 16 (not from/to Japan though)
I would expect that you should have no difficulty traveling alone out of country; unless you have a criminal record that prevents you from leaving the country. Years ago, we had a 17 year old exchange student live with us for a year. While she would have left the country with parental permission, it’s my understanding that she didn’t have any troubles leaving Japan. Does the Japanese government have a website that you could refer to? Good luck!