Is a firmware update required to read APFS volumes in Startup Manager in not so young Macs?
When High Sierra is installed, any necessary firmware updates are applied. The has to be true, because any Mac built before APFS would not be able to boot from a APFS container partition without a firmware update.

However, even if a Mac as the firmware update which include APFS, this does not automatically mean that versions of macOS (OS X) prior to High Sierra would be able to access APFS volumes. Apple would have to provide an APFS software update for older versions of macOS (OS X). I do not believe Apple has done this.

This is also true for Windows users. Many Ask Different users have reported failures to read APFS volumes from Windows and the inability to boot back to macOS High Sierra directly from Windows.

As for your assumption. I can not provide a proper answer since your reference to a Startup Manager is ambiguous. Each Mac has a firmware Startup Manager, but I think you are referring to the  Startup Disk pane from the macOS (OS X) System Preferences application.