An article in the New York Times points out that Live Free Or Die Hard is the only action movie made out of brick-and-mortar being released during the 10 week main stretch of summer. A 20th Century Fox executive says there's a dearth of "real things" in the summer movie market, and he thinks an action movie that makes minimal use of CGI will make a big impression. According to the article, that shot in the preview of a car being launched at, and demolishing, a helicopter was actually accomplished by launching a car at a helicopter.
In spite of that, though, Live Free... is still likely to be PG-13. The Times also reminds us that the Bourne Identity and Supremacy were PG-13, and still maintained a serious, adult tone (even so, The Bourne Ultimatum is holding off until August). It seems disingenuous to me to "challenge the summer drift to family-friendly fantasy and cartoons" with a PG-13 movie, but I guess it's not my $100 million.
It seems obvious to me, though, that the audience primarily interested in another Die Hard movie is well over 18 years old, anyway. The online film writer Vern addresses this better than I could in this article from Ain't It Cool News.
Also, Bruce Willis was 33 when he made Die Hard.