Tire companies will tell you to replace them within 5-7 yrs, but they are assuming the car is driven, and is outside in the UV from sunlight a lot. If you car is a garage queen, and never sees the 'light of day' (ie. UV rays), then the tires are probably OK for reasonable short distance driving, but I don't think I'd go racing or long distance high speed driving with them...
(I've got some Michelin X's on some older cars that stay in a basement garage (no windows), which still look good even though they are over 30 yrs old, and the original spare tire in my '57 Bel Air doesn't show any dry rot or cracks either...
Gary