DataCAD is backwards compatible, but it is impossible to be forward compatible. The file could be opened by DataCAD 17 (or 16) and then Save As to a v13 AEC file.
If you are saving from v16 to a v13 file that is not opening in v13, you might try a purge on the file in v.16 first and then save the purged file down to v13. If that doesn't work, you would probably have to send the file to Datacad's tech support for review for a small fee if you are not on their support subscription plans.
Thanks! - Josh Do. Or do not. There is no try. Josh's Digital Downloads is come back online soon. Stay tuned. (soon is a relative term)