At the Mojave event it's 5 years. Your personal risk tolerance will probably dictate how fresh your tires should be. In a stock GT running speeds up to 180 mph I would be comfortable with 5 year old date code brand new tires. I would not be comfortable with tires that have been run several thousand miles over 5 years as undetected road damage might compound their age. Running above 200 mph I start every Mile event with brand new fresh date code Hoosiers.
Running at these insane speeds with no roll cage or drag chute, I believe the most likely cause of a fatality is tire failure. A blown tire at 220 mph without a cage would probably not be survivable so fresh undamaged tires seem like a pretty good idea. The Texas Mile crash in the video link below was caused when a rear tire deflated after centrifugal force at 220+ pulled the tire bead off the rim. This Corvette had a stout full roll cage and the driver survived.
http://www.youtube.com/watch?v=1l6TW6OCpc4
Chip