Shelf Life of [Spare] Tires?

Had a flat tire on the return from the New Castle Regionals (USA) last
month and had to use the spare tire...the *original* (ca. 1992) tire
that's been inside the trailer the entire time. It looked brand new but
I took it easy driving back anyway. Now I'm wondering if a tire kept
out of the sun and off the ground has a finite life?

Google found some recent references to studies, with several cautions
that tires more than six years old should be replaced. Most refer to
tires on the vehicle but a couple related to tires stored in a
warehouse for over 10 years, then sold (with subsequent failure due to
"glue" weakness).

Anyone have any expert knowledge of whether the spare tires we carry
around for many years inside the trailers are still safe even though
they've got zero miles on them?

Chip Bearden
ASW 24 "JB"