Responses
The usual schooling in Japan (and korea) is 6 years of primary school (from the age of 7 to 12), 3 years of junior high school(from the age of 13 to 15), 3 years of senior high school (from the age of 16 to 18), and 4 years of university education(from the age of 19 to 22).
they are most likely to be 15
So they're 15 ?!

"Then in the future we must go to the same high school, the same university..." http://www.mangago.zone/read-manga/19_days/bt/c161/pg-6/
does this mean that they are in middle school? (〜 ̄△ ̄)〜