Yes, you can merge an old Outlook 2013 PST with Outlook 2021 and the easiest way to do it, especially if you have a large or multiple PST files, is by using the DataVare Outlook PST Merge Expert Tool. This application allows you to merge an unlimited number of PST files from any Outlook version including 2013, 2016, 2019 and 2021, without causing corruption or data loss. It combines all mailbox items including emails, contacts, calendars, tasks, notes and folders, while keeping the original structure intact. You can use the merge procedure to combine whole PST files, select folders. The nicest aspect is that it works without Outlook and supports both ANSI and Unicode.