SPOILERS for book 7
. . . These books ceased to be specifically for children halfway through the series; by Goblet of Fire, Rowling was writing for everyone, and knew it.
The clearest sign of how adult the books had become by the conclusion arrives — and splendidly — in Deathly Hallows, when Mrs. Weasley sees the odious Bellatrix Lestrange trying to finish off Ginny with a Killing Curse. ''NOT MY DAUGHTER, YOU BITCH!'' she cries. It's the most shocking bitch in recent fiction; since there's virtually no cursing (of the linguistic kind, anyway) in the Potter books, this one hits home with almost fatal force. It is totally correct in its context — perfect, really — but it is also a quintessentially adult response to a child's peril.
Read the rest of King's excellent analysis.