Not only is it important for seo how long the domain has been registered for, but what is also important is the length of time there has been content on the site. The age of links to your website is also something which is considered.
The reason the age of the domain is important is because it is a way of filtering out spam sites which are here today and gone tomorrow.
Another important thing is how long you register your domain for. If you register it for 5 years then you will have better results than if you only register it for 1 year at a time.
Here are a couple of articles you may find of interest which should answer the question in more detail;
http://www.webconfs.com/age-of-domai...-article-6.php
http://www.10e20.com/2007/03/21/why-...rtant-for-seo/
Hope this helps
