You cannot think of Gmail as being in folders, literally. While Gmail labels end up being shown as IMAP folders, that's not what they are in actuality. Gmail labels are, effectively, collections of symbolic links, and like symbolic links, the only time something referenced by them is actually deleted is when the final instance (the actual message/file) is deleted from the only location where it resides.
But, you can also select messages only older than a certain date and that are only in Inbox and have no other user defined labels applied:
in:inbox has:nouserlabels before:2010-03-01
This should return ONLY those messages in Inbox and with no other labels applied that are older than the date specified.