Harry Potter Mystery: How Could Almost Every Sixth-Year in “The Half-Blood Prince” Turn 17 by April, When the Cutoff Wasn’t for Several Months?

Everyone who is familiar with “Harry Potter” knows that a young wizard or witch can start Hogwarts when he or she is 11 and is expected to attend 7 years there. That means that by the time a student reaches his or her 6th year, he or she will turn 17 either during the school year or summer.