If it's just a trail rig with no high speed use, go for it. I had some tires on a previous car that looked great, no cracks, lots of tread. They were 12 years old and I had a blowout on the way to work, in the dark, on I-40. I pulled off at 12th street and didn't get a single bum to offer help me change the tire. Thankfully it was a rear tire and a small sedan that was already close to the ground so I was able to just slow down, pull over to the shoulder and put the hazards on. I don't keep any tire older than 10 years on any vehicle that has people in it whose lives I value. Compare the cost of new tires to the value of a loved one.