Somewhere on the side wall of you're tyres you should find a date code for when the tyres were made. If more than twenty years old cough up some pennies and go and buy a new set.
In regards to Ferrets I changed my tyres because they had cracks in them. I thought that if my missus was in the turret and we had a blow out, flip over and a death because I didn't change the tyres I wouldn't be able to forgive myself. Not worth it, not bothered if they don't look correct for the perfectionists or the keep them original lobby its my missus in the turret.
My two pence worth