There's no reason why they can't find talented 18- to 20-year-old actors to play angst-ridden roles. It would make the stories that much more credible as the actors age in the course of the series length.
The monstrous murder of master PR agent Ronni Chasen has put Hollywood's hard-boiled honchos back to the business of watching their own backs, just as they did after the Manson clan's murders in 1969. Once again, everyone is paranoid.
In its finale, Supernatural delivered not only one of the finest hours of network television of 2010, but it also supplied one of the most intellectually and satisfying closures any sci-fi or horror series ever broadcast.