Yes, it’s too old to run High Sierra or Mojave.Clicking on "About This Mac" it says mid 2011. As I recall, I purchased it Summer/Autumn 2011. That makes it too old then? Will I end up with problems syncing my iPhone with it in future?
Not unless you upgrade to a new iPhone in the future that needs a newer version of macOS to backup and sync. As long as you keep your present iPhone this won’t be a problem.