I was active on a Goldwing forum about 20 years ago when this was discussed at length. Some "experts" stated that tire compounds changed over time even when stored in a dark closet (as some had done).
But many agreed that you could compare new tires with aged and it was apparent that the old tires had hardened somewhat. Some stated that they mounted new tires that seemed slicker than they should have been (after break-in) and that the date code proved they were old.
It doesn't matter with auto tires so some dealers don't rotate their stock as they should.
The bottom line is that we should mount the newest date we can get and I remember some saying they'd not mount tires more than 5 or 6 years old. And good tire dealers would take them back.