While there may or may not be any definitive rules regarding this issue, it occurs to me that tires are something you wouldn't want to cheap-out on. For me, the question isn't "How old can my tires be?', but "Do I want to outlive my tires?"
But then, I'm not into rollcage testing.
On top of that, if you buy your racing tires now, who's to say that a rules change in the next 4 to 5 years won't make your tires illegal? Then you might have paid for 4 expensive racing tires that are of no use to anybody.
I think Interested Bystander has the right approach. With my SCCA and dirt track buddies, they get some old skins that are the same size as what they intend to race on in order to build their cars, and then get something new (or newer) when they're ready to race. An old, cheap set might be the way to go for build-up, but do you really want to be the guy on YouTube who throws a shoe at speed?
And at the risk of sounding selfish, I know I wouldn't want to be the guy patiently waiting behind you in 100+ degree temperatures while 4 to 5 years of your hard work and countless thousands of dollars were being painstakingly dragged off the lakebed because you got a good deal on tires in 2007.