How much important about how old are our blog? If you know some websites that selling products in internet, they usually give in their service about their web existence. So, for some bloggers, the age of their blog is one of thing that make them proud in blogging, besides they can get more higher pagerank potentially.
There is simple way to check blog or website age, one of the website is this: http://whois.domaintools.com/ , we can use who.is feature to check any informations about blog and website. If you use deafult adress of your blog like mine, it won't work, but if you have your own domain or using self-hosting, you can see the information.