Domain age is important to start any website. Consider that you are creating any new website. The domain name is new so no one heard of that and not indexed so if you would search you wouldn't find anything. After some days your site visible, but the progression is going very slowly because of your site not crawling by the search engine. If your domain is old, then search engine indexed your site quickly. It is an important element for ranking in search engine optimization.
To know the age of domain visit Whoisxy.com here it's display the domain name Creation Date and Expiration Date .To know the Domain name age use the calculation below.
Domain name expiration date -Domain name creation date=Domain name age.
For Example:Domain name Expiration Date is 2015 ,Domain name Creation Date is 2010.(2015-2010=5).So current age of the domain name is 5 years.