I have always considered tires that are 5 to 6 years old to be "Stale".

That said, the set of Nokian Hakkapeliittas on my 7 passenger are 6
years old and the ones on my '85 Westy are 5 years old.  The tread on
both (I use them year 'round) is still really good.  I'm having a hard
time wearing those suckers out.  I'm afraid, at some point, I may have
to replace them due to age rather than wear.  :(

Mike
Houlton, Maine