A tire out of the mold with mold release and 'sealed' surface rubber is in the category called shelf (or storage) life.
A tire that has been inflated, worn in, and exposed to the environment is in the category called the service life.
Shelf life plus service life equals total life.
Service conditions such as loading, heating, curb jumping, under/over inflation, etc affect service life, as does the quality of the materials being used in the mfg of the item and the mfg processes applied to the item.
Indeed, the rubber continues to be affected since vulcanization never really stops, but once the rubber and the internal belts are loaded by mounting and pressurizing and driving, the composite product begins to degrade from cyclical loading stresses causing such things as polymer chain interruption/cleavage/yielding, and environmental degradation.
A poor storage environment and a high duress service condition would likely be the basis for the most conservative assignment of these suggested limits but the inherent variability is huge, thus the decision must rest with the operator of the tire given the effect of the consequences if these statistical limits are exceed.
Maybe one day tires will be stamped with the date they are placed in service and then the clock tickers that want to waste resources and sell new tires will be satisfied.
Hassle by DOT whether they are correct or not, an accident where the blame can be shifted, cost of breakdowns, etc etc are all things to be considered in such a decision to use such an item.
