8-9 works fine, but he'd have to come downstairs after the parents are dead, otherwise actually seeing my guy slaughtering them may break him worse than just finding their bodies. No problem with right before his birthday. Also, I was thinking, non modern setting? I'm a fantasy buff myself, but for this I can go as late as the 1800s? If earth at all. I always have fun making new worlds, but if you'd rather not, somewhere in or before the 1800s works for me.