Shanky,
I actually meant you have to wait 6 months (if you are an "Indian" publisher) before you can use Adsense in a new blog, as per Google policies. Even though it is not part of the Google AdSense program policies, it is mentioned in certain places in official Google AdSense websites. However, in my opinion, Google does not count the exact number of days, but rather it uses its best judgement before approving AdSense application. If the site is not good enough, they could use the reason "Not 6 months old". If your site is a well developed site with good content, you could get approval even before 6 months.
The reason for this rule is to filter out "launch, cheat, earn, get disabled and repeat the full cycle again" kind of people. By enforcing a rule that the site has to do business for a good amount of time before they can do business with Google, they are reducing the bloggers who are involved in cheating, click fraud etc. Moreover, I don't think any blog is going to make any decent money during first 6 months, unless you are a big guy. Obviously, if you are a big guy capable of generating huge revenue, you have other means to contact Google directly!
As an AdSense publisher for last 8+ years, I have pretty good experience with AdSense system. I have spoken to several people in AdSense team and AdSense API team. They are really looking for people who have the potential and not looking for ways to keep people away. About 2 months ago, I did some extensive research on the requirements to participate in AdSense program and have published a detailed white paper here - http://www.zlobber.com/resources/10019-Should-websites-blogs-wait-months.aspx