This does not have to get complex. A fifth grader is typically ten or eleven years old. The concept of “ elementary” school could very well be vastly different elsewhere but the anime does not go deep into cultural customs and in general the idea is that the main guy is YOUNGER than the teen. Plus in the anime there are other fifth graders like him who are smaller, meaning that even in Japan and elsewhere 5th grade or elementary means those who are still children, NOT teens.

This does not have to turn into a discussion of whether or not I am writing to purposely neglect a worldwide audience. You are reaching. Just enjoy the article/anime... or not.