How does Outlook 2019 determine that a OST file is old?

How does Outlook 2019 determine that a OST file is old?

I have a user that is connecting to Exchange 2013 with two different laptops and I'm getting the message on one of them that "The .OST file is old".

Any ideas or suggestions other than to erase and let Outlook rebuild the .OST file.

Thank you,

Bart D. Hull