First of all, I would like to apologize to all my readers for not updating my blog for a very long time.
I had recently gone through a video in Google WebmasterHelp where Matt Cutts answered for a question on determining domain age. This is something most SEO experts would be aware but still I feel it is worth sharing coz still few feel that it is whois data that is considered for domain age.
In the video, Matt Cutts confirmed that Google determines the domain age of the website from the date when the crawler first see the link to a website. Google has filed a patent back in 2003 on Information retrieval based on historical data  Certain signals may be used to distinguish between illegitimate and legitimate domains. For example, domains can be renewed up to a period of 10 years. 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.
Adding to this the video discusses the importance of domain age with website authority. There was a time when spammers used to register domains for a limited period so when you registered your domain for more than a year it is less likely to be considered as SPAM. Does this work still now, is open to argument.
It is important to understand this but still it is one among the millions of factor that Google applies to a website.