From when i spent some time researching various info & summarizing all that i found regarding topic, tires can last unexpectedly long .. but if they are unused and stored in proper environment. One can buy 5-7 years properly stored tire just fine, it won't be degraded more then 5-10% (just that tire development process never stops and obviously new tire models/generations might be noticeably better). Hence sometimes it might be good deal to buy "new/unused" "old" premium tires.
Just that .. if you had used them, even with little mileage, "clock starts ticking". Tires, when driven, mini deform on each rotation many times. Some additives to rubber start to dry out, rubber chemical compound changes/degrades. Water / sunglight / ambient temp fluctuations, thermal cycling when driving. For mounted and started to be used tire i tend to not use them >3 seasons even if there is lot of thread left. There are some shops that buy/sell used tires, or one can put up ad and sell them used for someone .. one can also drift them down quickly on track to justify throwing them out
, but if one cares about safety/performance one shouldn't save on most important aspect/part of car affecting that all.
Paul Walker crashed in Porsche Carrera GT due it having 9 year old just 3.5K miles driven tires. Certainly when one saves on not getting new ones, when old hockey pucks "still have legal thread wear left", crashes & dies, then one will get as famous as him?