I think because most of the meetings and most of publications are written at a 5th grade level. I can't remember where my sister got that info, but it's true. If you read the Watchtower or Awake, they offer very simple sentences and words... that anyone can understand.
They do start at the elementary level, that is why kids are supposed to be in the meetings.... the meetings never progress to University level. Are you kidding? I never remember a meeting being intellectually stimulating or complex, no, you just read the paragraphs and then comment on the article and everyone nods their heads. That is why kids that are like 5 yeards old can comment at meetings because they are very elementary.
Sorry, but the whole thing is like Sunday school.
But I agree that kids should be taught on their level... I don't know that many kids that can even sit still through a 2 hour disney movie and THAT's exciting!