A Third Grade friend recently picked up Harry Potter and fell in love with the books. He chewed through the first two. By the third book he slowed down and the fourth book ended up being set aside.
Why?
It is not his skill as a reader. According to all the numerology and lexical computations attached to literacy today, he reads on a 5th grade level.
My third grade friend set Book 4 down because emotionally he is a 3rd grader. This makes perfect sense to me. I sobbed when Serious Black died.
J. K. Rowling aged her characters and her writing style with each book. Her first readers matured a year or so while they waited for the next installment in the series. The following generations of readers get to consume the books at will. For the younger advanced readers, this can take them out of their comfort zone.
Anyone who has spent time watching children knows of someone who grew much faster than the others. The tall children have a hard time because physically they look a few years older than they are. But let a 4' tall second grader scrape his or her knee and you will see their emotional age.
There is not a direct correlation between maturity and physical height or numeric reading level.
For those readers who are not ready for the later years of Harry Potter, I heartily suggest "The Familiars" by Adam Jay Epstein. This is a growing series from the point of view of magical animals. Lots of fun Potteresque magic, solid world building, and lots of personal growth. No static characters for Epstein. This is a plus in my book . "The Familiars" will challenge the 3rd and 4th grade readers, build their vocabulary and entertain them.