I absolutely LOVE this movie. Ewan McGregor is by far my favorite actor of all time, and he performs exceptionally in this film.
As for what you should be mindful of when deciding if you should allow your child to watch it, there really is nothing more but a little language and a little violence. There is extremely mild nudity. It's nothing that hasn't been in any other PG-13 movie. The rest of the movie is just a guy trying to figure a mystery out. The main reason why you should pause for kids 12 & under is because they might not understand the political aspect. I watched this movie when it first came out, and, at the time, I was 10 years-old. I didn't get some political things and what they meant, but I got the main gist and point of the movie. Now, I am 13 years-old, and I can watch this movie without even having to think about the politics. I understand everything. So, I would definitely recommend this movie to kids 13 & up because it is a riveting film that definitely puts you on the edge of your seat just waiting for what will happen next. I would definitely show this to a child 9 through 12 years-old, explain some things and get their take on it, and then show it to them when they reach my age and see how much their views and understanding of things has changed.
Overall, this is one of my favorite movies of all time, and I strongly recommend it.