I run my 4/4 tyres at 23psi all round, and feel that this gives the best combination of sidewalk flexibility and steering lightness. An old set of Avons that have been run at less than 15psi should definitely be replaced as the sidewalls will have been overstressed in my view.
I have Vredestein Sprint Classics on my car. The rears are 5 years old, the fronts are 9 years old. the car lives in a dark garage most of the time, so they are not exposed to much in the way of UV degradation. There is loads of tread left on each tyre - 4mm or so. There are no visible signs of ageing or deformation. I took it out for a good thrash round last week, and I could detect no difference in handling or road holding from that which I'm used to. I could get the tyres to chirp when I wanted to, and they clung on just as tight as I wanted them to when cornering.
For a heavy powerful car with upwards of 200bhp that is driven hard especially on track days, then yes, changing tyres every 5 or 6 years is probably wise. But for a 4/4 or +4 used for summer cruising and touring, not so much in my view.
I will probably put a fresh pair on the rears next winter and move the existing rears to the front, but I am not going to stress over driving around on old tyres this summer. Yes, it's a risk, but as long as the tyres have been looked after and regularly inspected, I don't believe it's _that_ much of a risk. With a car that only weighs 850Kg or so, they are far less heavily loaded than their design capacity.
