I've been watching a lot of anime this year, and notice that the number of large boobs on high school, and middle school, girls seems proportionately out of reality. Very few are voiced by actual high school students. (most young boys are voiced by grown assed women)
As for your main question, it has to do with work schedules and child labor laws. Kids are only allowed to work a set number of hours a day.
Caption this Meme
