Demographics has always had a larger bearing on crimes of violence than we'd like to believe. Thomas Sowell has written extensively on "crime waves" that peaked just as the most-susceptible youths were coming into their most-productive criminal years (about the age of 19-25, if I recall).
Besides this, another very interesting theory has been put forth to explain in part why crime has subsided. And it has nothing to do with gun laws. Rather, abortion laws:
I will have to think a bit about that article. Thanks for posting the link.