i had 5 year old tires disintegrate on me while going 65mph due to extremely poor maintenance from the previous owner. I've been told after 6 years you should change your tires.
in 2002 i bought a car with original factory tires from 1995, it didn't even occur to me to change the tires they had tons of tread left on them and the car was very low mileage. I drove on them for another 6 years where i put maybe 8000 miles on the car. Then the car was parked for over a year when i took a real good look at the tires there was 1/8" cracks all through the rubber and i could see fiber mesh inside. I knew i was an idiot for going that long between tire changes. I bought a new set of the same tires from the dealer and what amazed me first was how much the original tires had Shrank compared to the new tires of the same model the diameter was at least 2" shorter on the 95 tires verse the 2010 tires. felt like i was driving a truck the first time i took it out on the road.
having had 2 run-ins with the 'hey dummy change your tires' after a few years fairy I've changed my tire maintenance routine. A long time ago i didn't pay attention to the tires other then check the tread depth, now i realize tread depth is the last thing to look at. Age first, if a tire is in perfect condition and 8-10 years old, i will replace it.