On my system, I have autoarchive set for all messages over 3 months in age. Today I find that messages more recent than two years ago do not, in fact, appear when I list or search them. That is, in Outlook's various folder views, no messages more recent than 2016 are shown, nor do any that I know existed and should have been archived appear when I use either basic or advanced Find.
It is certainly possible that this is related to the file being huge, 3.6 GB. If so, suggestions on reducing it (copy thousands of messages to a new PST?) would be welcome.
I did run the scanpst.exe tool, but this had no effect.
I have already created a new archive PST and switched my future autoarchiving to that file.