Boondock Saints? I usually assume that most geeks have seen this movie. I think it's one of the best films ever, even though it starts a bit slow. A 15 y/o is likely old enough to see it.

Basically, it's about 2 Irish guys who do vigilante killings in Boston and has Willem Dafoe as a wonderfully arrogant FBI agent.

If you watch it, I'd love to hear what you think of it.