The other major thing that makes this series appeal to an older audience is the character design, art style, and setting. Although Light is only a High school student you are able to forget this because he is so tall and intelligent that he seems like he could be in college and this gives him a bit more credibility with Older audiences. He also rarely appears in a school environment past the first episode. He is more often found in a setting amongst adults. As far as the art style in this anime, it is very dark. A grey scale color pallet pervades every scene of the anime and the characters really only were black white or grey. This dark color scheme separates this anime from other shonen anime with extremely bright happy colors everywhere and allows it to seem more serious. Thus appealing to an older audience. In fact Madhouse uses the same color scheme here as they do i'm many other anime such as Shigurui that are definitely not for children.
I am impressed by Death Note's success at appealing to a large audience. The varying levels of depth in the show are genius and leave people of every age group with something they can enjoy about the series. Maybe the success of this anime will lead to similar anime being created in the future that allow multiple age group of people to connect and discuss based on common interests.