What prevents you from opening those messages?

The messages are still in the Inbox list. Messages older than 16 days will not reopen. These messages opened when newly received but now will not open. I used to be able to reopen messages from any date. Is there a historical setting that may have been adjusting telling Thunderbird to operate this way? Maybe it has to do with message compaction?

will not reopen.

What does 'will not reopen' mean? You double-click the message in the thread pane and nothing happens?

What's so hard with explaining what's going on? Mind you, you're the only one seeing what's happening on your screen.

I double click on thread pane. Message is identified but no content. Seems to be date related.

It sounds like your mail files are corrupted. First of all create a backup of your Thunderbird profile folder. https://support.mozilla.org/en-US/kb/profiles-tb#w_backing-up-a-profile

Then try to rebuild the index file of the troubled folder. Right-click the folder - Properties - Repair Folder

Note: depending on the corruption this may erase messages from the affected folder which cannot be recovered anymore. In that case you'd need to restore them from a recent backup done prior to the corruption.

Corruption is often caused by anti-virus software messing with Thunderbird mail files. It is therefore recommended to create an exception for the Thunderbird profile folder, so that the real-time scanner won't attempt to scan the profile with your mail.

For more information on the profile location see http://kb.mozillazine.org/Profile_folder_-_Thunderbird

Also see http://kb.mozillazine.org/Keep_it_working_-_Thunderbird