If you have Order object then you can get Guest Customer dob from getCustomerDob()
.
In ,magento database you can get guest customer dob from sales_flat_order
table's customer_dob
column.
Suppose,you have order then you can get guest customer dob from
$OrderObject=Mage$OrderObject = Mage::getModegetModel('sales/order')->load('Order_Id');
$DOb=$OrderObject$DOb = $OrderObject->getCustomerDob(); // yyyy-mm-dd hh:mm:ss