I know this sounds strange to the guys that make 100s of passes a year, but what I am asking is, are drag slicks no longer safe to use after 6 years ?

I only have about 17 runs total on my car and tires, but they have been on the car a long time.
There are no signs of dry rot and they rarely have been in the sunlight.
What is the life in years for these tires ?
Thanks,
Mark