It completely depends on the child and what they're used to. As I said I took my 6 yr old and then my 5yr old nephew.
My dd comes to the cinema with us most weeks, she has 2 much older siblings so is used to watching their choice, she's watched all the twiglights, hps, saw the avengers, iron man, and various romantic comedies. She's been coming with us since about 4, she knows to keep quiet, if she doesn't like it she just covers her eyes but she has never been scared to the point of getting upset.
My nephew fell asleep in Spider man, I don't think it fazed him at all.
Obviously if your child has only watched ( am struggling for examples here) but say postman pat then you stick them in front of the final Harry potter it isn't going to go well. It depends on lots of variables.