So, Firstborn (age seven-and-a-half) decided, when he turned off the video he was watching in his room, to sit on my lap and watch the video I was watching in the living room. This turned out to be Cowboys and Aliens, so he was treated to the big final battle and the exploding alien spaceship and whatnot.
This is of interest mainly because he had decided not to watch it earlier because it was too scary. Apparently the mental effort involved in Not Going To Bed Yet renders scary things a lot less scary.
I figure in another two or three years he'll be ready to watch Alien with me. At least, that was about the age where I first watched it with my parents, and under rather similar circumstances. Scary movies always > bedtime.