/jp/ - Otaku Culture

Loli is essentially just a blanket term for body type (and possibly personality type if played straight) and whether they're grade schoolers, middle schoolers, high schoolers or ロリババア is largely irrelevant, though all of those can be played as different kinds of fetish fuel.

As for eroge I think grade school is used more than middle school simply because the former reinforces the whole age taboo idea, and can also utilize tried and true elements like ランドセル more. End years of grade school is also roughly the time frame where pure kids first start finding interest in the opposite sex. Most people would likely find kindergarten too young.

High school on the other hand is the mainstay of eroge industry, thus you naturally also see a lot of high schooler lolis. I don't think eroge devs are consciously avoiding middle school, it's just that it doesn't have the same glamour of grade school most lolicons long for, nor the vanilla experience of high school. It's simply a case of pandering to the majority consumers and that's why middle schooler heroines are so rare.