From what I've seen most of the younger generation has a mobile phone and not a land line. To me this puts a skew on their numbers as it excludes a voting block.
About Methodology in Detail
Sample for our surveys are random digit samples of telephone numbers selected from telephone exchanges in the continental United States. The random digit aspect of the sample is used to avoid "listing" bias and provides representation of both listed and unlisted numbers (including not-yet-listed). The design of the samples ensures this representation by random generation of the last two digits of telephone numbers selected on the basis of their area code, telephone exchange, and bank number.
For more
http://people-press.org/reports/methodology.php3