My hubby was Michael last year, and we had a 6 or 7 year old that knew who he was, and kept coming back asking if Michael would be his friend, could he hold the knife... 
But then I've got friends that showed their toddlers horror movies and are completely unimpressed by the gore and violence in them now... but I think that's too young in my opinion.
Your stepson is 9? If he's not a timid, shy child, I'd say he's fine to see it. It really depends on the individual kid - some are too sensitive at 12, some could be ready and raring to go at 8. But don't forget about the sex stuff in there... that combined with the slasher/scary stuff could be a bit much for kids under 13.
But then I've got friends that showed their toddlers horror movies and are completely unimpressed by the gore and violence in them now... but I think that's too young in my opinion.
Your stepson is 9? If he's not a timid, shy child, I'd say he's fine to see it. It really depends on the individual kid - some are too sensitive at 12, some could be ready and raring to go at 8. But don't forget about the sex stuff in there... that combined with the slasher/scary stuff could be a bit much for kids under 13.