Google favours older domains for a number of reasons:

1. Well-established sites that own older domain names are often more reliable sources of information.
2. Spammers utilizing blackhat SEO techniques register and discard domains quickly in order to not get caught and legally penalized.
3. Due to the number of blackhat SEO and spammers, new domains are questionable until there is evidence of stability.

Google’s patent “Information Retrieval Based on Historical Data” clearly states that Google does take into account domain registration and renewal dates. The following quotes are from Google's search criteria documents:

• "e;...the date that a domain with which a document is registered may be used as an indication of the inception date of the document."
• Certain signals may be used to distinguish between illegitimate and legitimate domains... Valuable (legitimate) domains are often paid for several years in advance, while doorway (illegitimate) domains rarely are used for more than a year. Therefore, the date when a domain expires in the future can be used as a factor in predicting the legitimacy of a domain and, thus, the documents associated therewith."

A domain name may be used illegally for spamming and other black hat SEO purposes and then sold to a new, law-abiding owner with no change of the domain registration date, and without the new owner's awareness.

Domain names can sometimes be parked, that is registered but never used for a long time, and therefore have not provided evidence of the site being a quality site.

New domains are often used by highly established companies will often be successful using a 301 redirect from an older domain that has already proven itself and combining it with a very optimized campaign.