rallyesax's post in Winter tyres - how old is too old? was marked as the answer
Truly, winter tires are at their best for 1 year. Then they are still performing very well for 2 to 3 more years. I personally change them after 4 years maximum.
The compound does change too much after this period of time even if stored in the best way possible in my opinion (wet performance mainly, then snow)