Premiere 5.02 crash when editing

you need to tell us what year the imac was released theOS version and the graphics card .  You can find this info by going to the apple menu and going to "about this mac."   You should check this info against the minimum requirements.

https://helpx.adobe.com/premiere-pro/system-requirements.html

Also, what are your source properties and sequence settings?    16 gigs of RAM is not alot particularly if you're working with 4K or greater footage.   If this is the case, you might investigate a proxy workflow.

And it's possible that using an earlier version of premiere might be better as hardware requirements usually get more demanding with an update.  You can still install premiere 14 on your system using the creative cloud app.  Unfortunately Adobe no long makes earlier versions available...