I also generally do not put anything older than 5 years on my bike, considering that the front will be gone in 2 seasons at most and the rear within 3/4 of a season. A 3 year old tire is usually the max age you will find a brand new one at regular price at a shop/store/dealer. Anything older should be heavily discounted.
Yes, storage is important. UV kills tires, heat is not the greatest either, nor are ozone, any kind of weight/pressure and moisture. Cold doesn't do much to them. Best way to store tires is out of sun, cool and dry, in a plastic bag.
Unfortunately, most of the time, there's no way of verifying how the tires were stored, therefore try to purchase newer DOTs whenever you can.