Dear singhbadshah In some locations, including China and India, Google AdSense require publishers to have owned their sites for 6 months. They've taken this step to ensure the quality of their advertising network and protect the interests of their advertisers and existing publishers. I suggest you to head out through Is my site eligible to participate in AdSense? article inside AdSense Help › Help articles › AdSense basics › Signing up. I believe it’ll help you to find an answer/ solution to your question/ problem easily. Towards your success,
if you are in india, then your site needs to be atleast 6 month old before getting accepted to adsense program. once you are approved, then you can use adsense on all sites, including new ones
Traffic and content determine this thing. A website among the top 100,000 websites in alexa ranking will be preferred by Adsense
Well, like people have said, in places like India your site must be 6 months old to retain content worthy material. In other places like the US there is no need for the site to be of any specific age.