What hoofhearted says is spot on.
Having said that, on another (Australian) board I frequent, there is a fellow who deliberately asks for the oldest tyre the shop is carrying. This is because the rubber does indeed become harder, and thus wears less. One downside is that it also offers less grip, and definitely shouldn't be pushed in the wet. The poster who does this commutes a large distance (20,000km?) every year and buying these older tyres helps offset the milage costs, due simply to their hardness giving long service life.
If you're planning on doing anything more than commuting, look for a decent new tyre. It doesn't have to be some $600 hyper-sports tyre set, but just so long as its new and suits your purpose (tourer, street, sports-tourer, sports etc).
Cheers - boingk