The gore factor is minimal and the language is very tame, I heard the "F" word once.
I'm going to take him, if he wants to see it- I think it's something he needs to understand - exactly what happened that day.
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I bet a side by side comparison with any modern PG-13 movie would have more curse words and/or violence than United 93. I also bet that the R rating was issued to keep away family viewers. I am very protective of the minds of young people and believe that this movie, while powerful, is not too powerful for a teen-ager. It shows the work of ordinary people deciding that they are in a life-threatening situation and have to help themselves as best they can. A lesson that teen-agers are old enough to learn.
It was probably uttered at least 20 times, each time was appropriate.