It is actually an official rule inherited from back in the days when JR companies were government entity called Kokutetsu.

Child fare on JR applies to elementary school (1st – 6th grade) students. 7th graders and up are adult fare on JR.

I know those of you always have to find what is wrong with me.. , but this rule was established when JR was government entity. Elementary school in Japan is compulsory education, required by law. In Japan 1st – 9th grade is compulsory, required by law. You may say what if my child does not go to an elementary school? As a government agency (back then JR was) the interpretation was that if your 8 years old child is not attending an elementary school then that is illegal.

I even heard an argument back when JR was a government entity that “child fare” on Kokutetsu was meant as a student discount so if a child is not attending an elementary school then a child is supposed to pay an adult fare.

I know you will be all over me saying then how about junior high, high school, and university students? Aren’t they students? Junior high, high school, and university students receive a voucher called Gakuwari (学割) issued through a school which provide discount on a base fare on travel of more than 100 km on JR lines. And, yes, there is no age limit on Gakuwari voucher. As long as a students is a full time students then even 45 years old attending an evening high school is eligible for Gakuwari voucher.