i forget the exact numbers, but its something like within 4 years its lost 50% . i know at the shop i worked at we had to get apprval to work on anything 5+ years old and 10+ we were not supposed to touch. rarely followed this. it depends on more than just age. mostly location and if they were kept outside (sunlight takes the oils right out of a tire hence the cracking and stuff you'll see in a sidewall). its scarier when you see codes that are from the 90's or before the DOT numbers were in their current format.