I have simply used shopping cart price rules by the following way.
first i had created product attribute named as days. Then I have set the value to that days textbox as programmatically as follows
require_once("app/Mage.php");
Mage::app();
$productCollection = Mage::getModel('catalog/product')->getCollection();
$date2=Date("Y-m-d");
foreach($productCollection as $_product)
{
$product = Mage::getModel('catalog/product')->load($_product->getEntityId());
$date1=$product->getCreatedAt();
$diff = abs(strtotime($date2) - strtotime($date1));
$years = floor($diff / (365*60*60*24));
$months = floor(($diff - $years * 365*60*60*24) / (30*60*60*24));
$days = floor(($diff - $years * 365*60*60*24 - $months*30*60*60*24)/ (60*60*24));
$product->setDays($days);
$product->save();
}
After that i have added that product attribute promo rules. Then i set the condition as if differerence between dates which are from product creation date to now is more than 15 days, to add discount to that particular product.