I was just speaking with my nephew about this. He's 12 now and he found PoA scary (he was hiding behind the cushions during Lupin's transformation), but he still really wants to see GoF, I hope he can handle it.

As the the movies progress, the certificate may raise judging on the content in the books. The duel in the bathroom between Harry and Draco in HBP for instance. It's just a guess...