what age do you graduate high school

Between the two Countries, US has a lot more choices in terms of Majors/Degrees, but are a lot more expensive than UK. (International Students cost even more)
Japan’s High School education system finishes at the age of 18. University is usually a 4 year course except some medical degrees.

Your school leaving age depends on where you live.
If you turn 16 between 1 October and the end of February you can leave at the start of the Christmas holidays in that school year.

References:

http://www.gov.uk/know-when-you-can-leave-school
http://www.kingscourt.org.uk/key-stages/