Well, its based off of the original Death Note manga, which ran in weekly Shounen Jump, and IS technically a Shounen series, meaning that someone had to have at least considered it to be kid-friendly enough to run in a Shounen Magazine. But, then again, there are a ton of shounen series out there, which are "technically" considered to be shounen, but really don't feel like they should be running in shounen magazines, at all.
At any rate, I wouldn't say that Death Note is very kid-friendly at all, myself, but in all honesty, I just found the movie to have such cheesy and over-the-top acting, that I really couldn't take it seriously enough to really call it "mature," either.