Answer by
LeheckaG (1826)
A few sources published "6 years" as being the "normal" shelf-life,
under cooler/darker (Northern) conditions "10 years" might be normal.
DOT sidewall tire date codes are:
(3) WWY (198Ys, 199Ys with a "triangle")
or
(4) WWYY digits (20YY's)
specifying the WW week and Y/YY year a tire was manufactured.
Primarily, air/oxygen and heat/radiation/sunlight degrade the tire rubber over time.