My son was hell-bent on seeing The Exorcist from an early age ... begged and carried on and we were, like, "No ... far too shocking and intense for a young mind!" I remembered writing the local ABC affiliate in Pittsburgh when I was a kid because the tv trailers for it scared the hell out of me (and they sent a note back on station letterhead apologizing).
We finally gave in when he was maybe ten and let him watch it. He was happy to see the actual upstream of decades of pop culture references, but when it was over he just sat there and said "that's it?" Completely nonplussed. Just went upstairs to play Minecraft or whatever.
He's home from college this month and it came up because of the new sequel coming out and he was very clear on why it hadn't phased him years ago: "You and mom raised me atheist. Why would I be afraid of demonic possession?"