It really depends on where you live and keeping in mind that not all "fifth grade books" are created equal. The Wimpy Kid series I would say not that rare. If it were Harry Potter or Chronicles of Narnia, I would say kind of rare. There appears to be an extremely wide range of reading ability in K through 2nd grade, partly but not completely correlated to "giftedness". Even among my own children, the range narrows and widens depending on exactly at which point you compare.