Are all the items stored in the same PST file or are the old items stored in
an archive PST file?

I believe Advanced Find will always search oldest to newest.

DickB,

It has been my experience that Outlook searches the .PST folder from start
to end of file, stores messages chronologically from when they were added to
the folder and not by the date.  I've been using Outlook for more than ten
years now, and my topical .PST files will have folders from different periods
of time.  When I perform a search but my view is organized by descending
order of sent date, the display will move up and down as messages are found
and organized by date.  The only way I know of to get the search order you
seek is to search for all the messages in your folder, organize them in the
find window, create a new FOLDER, then copy the messages to the new FOLDER.

In the early years, I wanted the same search results you do but was informed
you cannot do it and continue adding newer messages to a folder because of
the way outlook stores the information in a .PST file.  Since I am frequently
merging topical messages that span years into archive folders, their storage
often puts the oldest messages at the bottom of the file and Outlook has to
sort them out in the search window.